Political scientists are interested in complex concepts: democracy, war, economic development, protest, or nationalism. To study them, researchers sometimes create original datasets that measure these concepts across multiple units (e.g., countries, provinces, municipalities). Constructing original datasets usually requires considerable resources, but the payoffs for the discipline as a whole can be large: these datasets eventually become public and enable not only their creators but also other researchers to study a wide range of questions.
Making appropriate descriptive and causal inferences based on datasets created by other academics, however, is not straightforward. It requires understanding how the dataset was constructed—how variables capture multidimensional concepts, how each dimension is operationalized, what information and sources were used, or what coding assumptions were made. Consider, for example, the concept of democracy. There is widespread agreement among democracy researchers that democracy entails, at the very least, two dimensions: competitive elections and mass enfranchisement. Despite this agreement, cross-national datasets that code democracy frequently disregard the “mass enfranchisement” dimension (Munck and Verkuilen Reference Munck and Verkuilen2002). Moreover, among those that do measure enfranchisement, some measure it continuously, while others use varying thresholds above which a country is considered democratic—for example, a majority of adult males must be able to vote in Boix, Miller, and Rosato’s (Reference Boix, Miller and Rosato2013) classification, whereas universal male suffrage or, simply, universal suffrage is required by Skaaning, Gerring, and Bartusevičius (Reference Skaaning, Gerring and Bartusevičius2015).
This example demonstrates a broader point: dataset creators have ample freedom to choose which dimension(s) of a complex concept to measure and how exactly to measure it. As a result, different datasets may offer variables that, despite using the same terms and referring to the same basic concept (e.g., “democracy”),Footnote 1 measure different dimensions of that concept, have different validity and reliability characteristics, and are collected in different ways. One downstream consequence is that a high cross-measure correlation is not necessarily ensured. Even in the absence of measurement error, two variables that appear to tap into the same concept may exhibit divergences because of seemingly trivial—but, at closer inspection, important—differences in how they were constructed.
To further illustrate this matter, consider the different patterns that emerge in figure 1 depending on which measure is used to capture four important phenomena: intrastate conflict, democracy, education centralization, and repression. The figure shows the global means of different measures for each of these concepts, using only country-year observations with data for all measures of a given concept. Still, the measures exhibit differences that may have meaningful implications for inference. For example, depending on which data source we use, the share of countries experiencing intrastate conflict in recent years oscillates between 5% (Haber and Menaldo Reference Haber and Menaldo2011) and 33% (Uppsala Conflict Data Program; see Pettersson Reference Pettersson2022).Footnote 2 Global democratic decline took place during the 1960s according to one democracy measure (Boix, Miller, and Rosato Reference Boix, Miller and Rosato2013), but not according to another (Regimes of the World, which builds on V-Dem data; see Coppedge et al. Reference Coppedge, Gerring, Knutsen, Lindberg, Teorell, Altman and Bernhard2023). The degree of education centralization varied more from 1945 to 1995 according to one measure (Education Policies and Systems across Modern History [EPSM]; see Del Río, Knutsen, and Lutscher Reference Del Río, Knutsen and Lutscher2024) than according to another (Varieties of Indoctrination [V-Indoc]; see Neundorf et al. Reference Neundorf, Nazrullaeva, Northmore-Ball, Tertytchnaya, Kim, Benavot and Bromley2023). Finally, the level of repression around the world increased (Physical Violence Index; see Coppedge et al. Reference Coppedge, Gerring, Knutsen, Lindberg, Teorell, Altman and Bernhard2023), remained similar (Cingranelli-Richards Human Rights Dataset; see Cingranelli, Richards, and Clay Reference Cingranelli, Richards and Clay2021), or declined (Political Terror Scale; see Gibney et al. Reference Gibney, Haschke, Arnon, Pisanò, Barrett, Park and Barnes2022) in 2011 relative to 1981, depending on the choice of repression measure. Our point is not that measures of the same concept (or at least similar concepts) should never diverge—justifiable differences in conceptual specifications, operationalization, aggregation, or other features can lead to different measurement outputs—but that both data creators and users must be mindful and transparent about the measurement process and its potential implications for inference.Footnote 3
Figure 1 Comparisons of Different Measures of the Same Concept
Sources: BMR (Boix, Miller, and Rosato Reference Boix, Miller and Rosato2013); CIRI (Cingranelli, Richards, and Clay Reference Cingranelli, Richards and Clay2021); CoW (Sarkees and Wayman Reference Sarkees and Wayman2010); EPSM (Del Río, Knutsen, and Lutscher Reference Del Río, Knutsen and Lutscher2024); HM (Haber and Menaldo Reference Haber and Menaldo2011); PTS (Gibney et al. Reference Gibney, Haschke, Arnon, Pisanò, Barrett, Park and Barnes2022); PVI (Coppedge et al. Reference Coppedge, Gerring, Knutsen, Lindberg, Teorell, Altman and Bernhard2023); RoW (Coppedge et al. Reference Coppedge, Gerring, Knutsen, Lindberg, Teorell, Altman and Bernhard2023); UCDP (Pettersson Reference Pettersson2022); V-Indoc (Neundorf et al. Reference Neundorf, Nazrullaeva, Northmore-Ball, Tertytchnaya, Kim, Benavot and Bromley2023).
Note: CIRI and PTS are ordinal variables that have been rescaled to a unit interval using min-max scaling to facilitate comparisons.
In this paper, we discuss the advantages, limitations, and trade-offs involved in creating original cross-national datasets for research purposes and distill lessons and guidelines for both dataset creators and users.Footnote 4 To do so, we draw on the collective knowledge developed by the creators of three different but interrelated longitudinal, cross-national datasets on education systems: the EPSM dataset (Del Río, Knutsen, and Lutscher Reference Del Río, Knutsen and Lutscher2024), the V-Indoc dataset (Neundorf et al. Reference Neundorf, Nazrullaeva, Northmore-Ball, Tertytchnaya, Kim, Benavot and Bromley2023), and the Historical Education Quality (HEQ) dataset (Paglayan, Reference Paglayann.d.). While all three datasets contain seemingly similar measures of education, they differ in many respects. This goes for easily visible differences such as coding de jure (formal-legal) versus de facto (operation-in-practice) features of education systems or relying on country experts versus in-house coders, as well as more subtle but consequential differences such as how they deal with uncertainty or what thresholds are used to establish coding categories. Our goal is to codify good practices and share tacit knowledge developed through the experiences of these data collection efforts made by different teams of researchers. We remark that not all of these practices or insights were obvious to us before embarking on the different data collection efforts. In online appendix A we give a more detailed overview of unanticipated challenges and how we changed strategies or adopted measures to mitigate them in the hope that future dataset creators may learn from our experiences.
By opening the black box of dataset creation, we hope to stimulate greater transparency at this stage of the research process. While the discipline has moved toward a norm of transparency in data analysis, a similar norm has yet to be developed regarding the process of dataset creation. Developing such a norm is crucial because, as will become clear in this paper, the choices made during the process of constructing new datasets can have far-reaching implications both for descriptiveFootnote 5 and causal inferences (e.g., Casper and Tufis Reference Casper and Tufis2003). To move the needle in this direction, we pay considerable attention to both the advantages and disadvantages associated with various data collection decisions. This is not because we believe that the latter are more prevalent in the datasets that we examine relative to other datasets, but rather out of a conscious effort to normalize the process of making various measurement challenges and trade-offs as clear and explicit as possible. The peer-review process and other features of academia may incentivize dataset creators to hide or minimize the disadvantages or limitations of their datasets, which does a disservice to readers, users of these datasets, and the research community more broadly. We hope that by reflecting deeply and being open about the limitations of our datasets, we can raise awareness about the inherent limitations in assembling and using any dataset.
Our main contribution is to develop a set of guidelines for dataset creators and users, which we summarize in table 2 of the final section, following a detailed reflection on how to collect data and its challenges. These guidelines are aimed at enhancing transparency, replicability, and valid inferences in the social sciences. Furthermore, our paper contributes to ongoing methodological debates in political science. First, Gemenis (Reference Gemenis2012, 595) argues that using secondary sources (e.g., party newspapers, leaders’ speeches, etc.) instead of primary sources (party election manifestos) to code the ideological positions of parties can increase nonclassical measurement error.Footnote 6 We reach a similar conclusion in the context of coding de jure education policies. Moreover, we illustrate a common trade-off that researchers face when choosing whether to rely exclusively on primary sources (reducing measurement errors) or whether to also use secondary sources (reducing coding costs and increasing coverage). Second, we contribute to the ongoing debate about the advantages and disadvantages of relying on factual data sources versus expert assessments (e.g., Knutsen et al. Reference Knutsen, Marquardt, Seim, Coppedge, Edgell, Medzihorsky, Pemstein, Teorell, Gerring and Lindberg2024; Little and Meng Reference Little and Meng2024). For instance, we highlight how different data types may have varying benefits and drawbacks depending on what concept or concept dimensions one aims to measure, as well as whether one aims to capture de jure or de facto aspects of the concept.
Background: Three Datasets, Same Topics, Different Methods
We begin by providing a brief overview of the three datasets—EPSM, V-Indoc, and HEQ—that form the basis of the lessons we draw in later sections for dataset creators and users (for detailed dataset descriptions, see online appendix A). These cross-national longitudinal datasets offer rich information about the content of education, teacher training and recruitment policies, and the distribution of authority over the education system. However, while EPSM and HEQ focus primarily on de jure policies, V-Indoc focuses mainly on what the contents of education and teacher recruitment look like in practice. The information used to construct each dataset also varies: EPSM relies on a combination of primary and secondary sources for 145 countries from 1789 to 2020; V-Indoc relies on country-expert assessments across 160 countries from 1945 to 2021; and HEQ, still under construction, relies exclusively on primary sources such as education laws, regulations, decrees, and national curriculum plans, and to date covers five countries over the past two centuries. Table 1 provides an overview of the datasets, including their key characteristics and main advantages and disadvantages.
Table 1 Advantages and Disadvantages of Different Data Collection Methods
These datasets illustrate a common trade-off between coverage in terms of country-years and potential sources of measurement errors and, hence, the precision of the data. For example, primary sources offer accurate data for datasets focused on capturing information about de jure policies, but gathering all the relevant primary sources can be extremely time consuming and may not be possible for some countries or periods. Thus, researchers seeking to enhance the accuracy of their measures may need to decrease the coverage of their sample. This trade-off is evident when comparing the coverage of EPSM, which combines primary and secondary sources, and HEQ, which relies entirely on primary sources. While data assembly took around 19–22 hours per country for EPSM, it has taken between three and six months in the case of HEQ.
Relatedly, while using secondary sources can enable dataset creators to expand the geographic and temporal scope of their dataset,Footnote 7 one downside of secondary sources is that the information they contain could be incomplete or inaccurate. In fact, in the process of assembling HEQ, the team discovered that the conventional wisdom inherited from influential studies about the history of education in some countries was not corroborated by actual historical records. These mistakes stemmed from a tendency in the secondary literature to assume (incorrectly) that a de facto education practice was grounded in a de jure policy or a tendency to focus on the most famous education laws and neglect lesser-known laws and regulations that nonetheless formed part of the de jure educational landscape. Indeed, early comparisons between EPSM and HEQ revealed some measurement errors (which were later corrected) stemming precisely from EPSM’s reliance on secondary sources and expert knowledge when education laws were not accessible. These sources could be influential but sometimes inaccurate.
While relying on legal texts helps us to measure de jure education policies, laws tell us little about whether these policies were, in fact, implemented. For information about on-the-ground education practices, we need a different data collection approach. Here again, a trade-off between breadth and accuracy arises. For example, one could obtain information about what children are actually taught in school based on classroom observationsFootnote 8 or by surveying scholarly experts who have in-depth knowledge of a country’s education system. The former will likely produce more accurate results, but conducting classroom observations is far more costly than surveying experts. Moreover, classroom observations allow us to gather data on current and future education practice, but we cannot rely on them if our goal is to collect data about the past.
Experts assessments are one approach for collecting data about past practices. By drawing on their in-depth contextual knowledge and evaluative judgment of a topic, country-specific experts—sometimes recruited locally from the country of interest—can offer guided insight into difficult-to-measure aspects of education systems, such as politicized teacher firing or indoctrination (Marquardt and Pemstein Reference Marquardt and Pemstein2018). However, there are also disadvantages to using expert surveys. First, expertise may also be time bounded; indeed, the reason why the temporal coverage of V-Indoc is limited to 1945 onward is because pilot studies revealed that experts did not feel confident coding their country of expertise further back in time. Second, experts may draw on cognitive heuristics when responding to questions (Weidmann Reference Weidmann2022), and some responses may reflect coder bias (e.g., Little and Meng Reference Little and Meng2024; nonetheless, this feature may also influence nonexpert coding; see, e.g., Knutsen et al. Reference Knutsen, Marquardt, Seim, Coppedge, Edgell, Medzihorsky, Pemstein, Teorell, Gerring and Lindberg2024).Footnote 9
Overall, the inherent tensions between breadth and accuracy (given resource constraints) and the choices made by each research team concerning which goal to prioritize resulted in EPSM and V-Indoc accomplishing a substantially broader coverage than HEQ in much shorter time, but at the potential cost of accuracy. As we discuss in the Advice for Dataset Users section, such features of the data and the implications of the discussed trade-offs should also be considered by data users conducting different types of studies; for instance, accuracy may be a relatively larger problem for single-country case studies, whereas smaller and selective samples may be a relatively larger problem for cross-country studies.
Another important consideration for data creators is monetary costs. While the assembly of any cross-national dataset is likely to demand considerable resources, the data collection approach chosen has consequences for costs. EPSM conducted all data collection in-house, hiring and training research assistants who gathered and coded primary and secondary sources and later discussed a final coding decision with the EPSM team. This resulted in an average cost of approximately $1,000 per country. V-Indoc relied on one postdoctoral scholar and multiple research assistants to identify and recruit country experts, recruited and compensated close to five experts per country on average, and paid for the use of the V-Dem Institute’s data collection and measurement infrastructure for an average cost of $2,000 per country. HEQ hired education historians from each country as consultants to gather all primary sources and to conduct an initial round of coding based on these sources; a research assistant then cross-checked the initial coding against the primary sources for all countries, which led to a back-and-forth with consultants before arriving at the final coding for an average cost of $7,200 per country.
Advice for Dataset Creators
When collecting and assembling datasets, researchers invariably face challenges pertaining to validity, reliability, transparency, and reproducibility, and need to make decisions to mitigate such issues. This section illustrates these challenges by drawing on experiences from, and comparing across, our three education datasets. On a related note, we discuss the practices and tools that helped us to mitigate these issues and try to generalize different insights through a set of guidelines for future dataset makers, which we detail and concretize further in online appendix D1 and summarize in table 2 in the concluding section.
Table 2 Guidelines for Data Creators and Data Users
Note: * Online appendix D1.1 includes a detailed checklist for best practice on creating codebooks.
Codebooks and Specification/Clarification
To enhance transparency, dataset creators must overcome the challenge that specific questions and question categories may have multiple plausible interpretations. A related challenge is that the meaning of specific terms (e.g., “public school”) may vary across countries and over time. Thus, dataset creators should pay particular attention to specifying their codebooks so that one minimizes the number of plausible interpretations per key term, concept, question, or category, ideally ensuring that they can only be interpreted in one way. While this might sound straightforward, our experiences with codebook construction suggest it is often hard to achieve in practice. Accomplishing unambiguous interpretation requires anticipating all possible interpretations of answer categories and possibly breaking complex questions up into two or more questions to mitigate multidimensionality.
Maintaining consistent definitions of evolving terms or even concepts and avoiding multidimensionality and ambiguity in interpretations are, as indicated, often surprisingly difficult. Indeed, these issues may even be hard to detect. Yet several (fairly straightforward) strategies can help to address such issues. Dataset creators should provide clear definitions of key concepts, clarifications, and even hypothetical or brief empirical examples to illustrate the coding procedure. This not only helps to ensure transparency to data users wondering exactly how questions and categories should be interpreted (or how they align with their specific research questions and contexts), but it also improves intercoder reliability and reproducibility and, crucially, ensures that the dataset provides information that is comparable across space and time.Footnote 10 Dataset creators should also invest considerable time when formulating questions and allow many people, including outsiders who may interpret questions very differently, to review the codebook. For example, the V-Indoc team took two years to develop the codebook (expert questionnaire) and relied on detailed feedback and advice from subject experts at multiple stages of the questionnaire development process. These experts also helped to map abstract concepts onto specific questions, which is often a key challenge when developing codebooks.
A complementary strategy is to pilot the codebook in a subset of (preferably quite different) cases to detect potential issues with how questions and categories work, and adjust the codebook accordingly. All three teams—EPSM, V-Indoc, and HEQ—followed this procedure and gained valuable lessons from piloting. For example, the EPSM team piloted an initial questionnaire on a dozen countries to assess the feasibility of collecting data in different geographic and institutional contexts. The resulting experiences—as well as subsequent experiences, after the main coding had started, as we detail in our online appendix D on hard lessons learned from our data collection experiences—were instrumental for altering or developing new answer categories, specifying coding rules of thumb for interpreting and scoring tricky cases, developing practices for references and for including justifications of coding decisions, and developing detailed coding instructions and materials for training research assistants. Similarly, the V-Indoc team met with the coders participating in eight pilot cases to discuss their coding experiences. These conversations enabled the team to identify questions that needed to be simplified to avoid being multidimensional. For HEQ, piloting in two countries helped to identify questions where the team had not anticipated the full set of possible answers, as well as additional strategies for documenting sources (via pictures) to ensure reliability.
As noted, specifying key terms and question categories and writing detailed question clarifications also enable users to understand better how the data have been produced and, thus, the dataset’s contents. Additionally, these strategies enhance intercoder reliability and, therefore, replicability (if the second coder aims to replicate the data construction effort) and dataset consistency (if different coders code different units in the dataset). Absent such strategies, different coders will likely rely on dissimilar heuristics when making coding decisions. In cases where multiple coders contribute to a dataset, this is likely to produce different patterns of missingness (e.g., because coders treat uncertain cases dissimilarly) and different uses of particular categories (e.g., because some coders have higher thresholds for assigning high scores than others). Insofar as coders are assigned cases based on, for example, their regional, language, or historical-period expertise, there may thus be systematic differences across subsets of observations that could correlate with other factors of theoretical interest (such as income level, state capacity, or democracy, which vary systematically across regions and periods). If so, this might contribute to biased inferences in studies using the data for operationalizing independent or outcome variables and studying their relationships with income, state capacity, democracy, or some other feature correlated with the former three concepts.
More generally, low intercoder reliability may cause additional problems for datasets coded by more than one person, so it is important to consider additional strategies for ensuring consistent coding across individuals. For example, the EPSM dataset relies on five in-house coders who coded different subsets of countries. All coders were in frequent contact with each other and the research team, which meant that several other strategies could be applied to enhance intercoder reliability. Some important strategies were (1) an intensive training scheme with repeated trial coding of the same cases to make sure that all coders understood the terms, tasks, and data sources similarly; (2) developing and updating a joint rules-of-thumb (RoT) document for tricky cases (e.g., where coding decisions indicated by the codebook were ambiguous), detailing how particular types of cases were supposed to be interpreted and coded; (3) active communication through a joint web platform and (sometimes) physical colocation when coding, allowing coders to discuss and find joint solutions to challenging cases; and (4) a second coder going through all original codings, with subsequent adjustments. These measures were intended to aid coders in having a similar understanding of terms and underlying concepts and applying similar heuristics (preferably made explicit in the RoT document) when approaching similar cases. Nonetheless, avoiding differential interpretations and uses of heuristics across coders is close to impossible to guard against completely, and such between-coder differences may lead to increased uncertainty and even biases, as noted above. Dataset creators providing coder IDs and explaining coding decisions for each coded observation may be one strategy for allowing users to assess and possibly reduce such issues in their analyses.
The country-expert-coded V-Indoc dataset relies on a Bayesian item-response theory measurement model to make estimates comparable across experts and countries. This model was developed for the wider V-Dem dataset to deal with several issues, such as experts having different understandings of questions and applying different thresholds when choosing between categories (for details, see Coppedge et al. Reference Coppedge, Gerring, Glynn, Knutsen, Lindberg, Pemstein, Seim, Skaaning and Teorell2020; Pemstein et al. Reference Pemstein, Marquardt, Tzelgov, Wang, Medzihorsky, Krusell, Miri and von Römer2025). The measurement-model method incorporates several pieces of information (e.g., experts’ coding of vignettes, bridge coding of selected countries and time periods, cross-coder divergences, coders’ self-reported confidence, and estimates of coder reliability), to adjust experts’ scores before aggregating them to the country-year level, which enhances the comparability, reliability, and validity of the estimates while also generating uncertainty measures for each estimate. In this process, the measurement model transforms experts’ original scores on an ordinal-level indicator to a (presumed underlying) interval-level scale. The latter transformation relies on nontrivial assumptions that are indicated in the codebook (alongside references to more detailed documentation) together with the measurement level of the variable contained in the dataset.
The latter point illustrates a more general one for codebook construction: indicator entries should contain precise information about scaling in order to provide users with the requisite information to avoid erroneous interpretations of scores and evaluate which kinds of analyses variables may be used for, among other purposes. We list this as one guideline for constructing codebooks, alongside several other pieces of advice indicated in this section, in table D1.1 in the online appendix. Scaling information is provided in the codebooks of the three education datasets, although the information is sometimes insufficiently specified or otherwise problematic.Footnote 11
Triangulating Sources
A common practice among historians is to triangulate information from multiple sources, which helps to acquire a holistic picture of the object of study, assess the reliability of different sources, and enhance confidence in our conclusions when multiple sources point in the same direction. While triangulation is often used by researchers relying on qualitative evidence (e.g., by combining interviews with qualitative document analysis), the last two points are also relevant for the construction of quantitative datasets.
For example, the authors of EPSM first collected secondary data sources on the history of education and other relevant sources to identify key legislation and obtain background information about the case. Afterward, the authors collected all available legislation online or through library exchange. When data sources diverged, the team established a protocol and guidelines for dealing with the divergence in their RoT document: if primary and secondary sources led to different coding decisions, primary sources were prioritized, and the level of confidence was also registered. If doubts prevailed after a second coder revised the case and checked intracoder consistency, the team met and discussed potential sources of coding disagreement and strategies for additional source collection.Footnote 12 The goals of this procedure and the wider triangulation strategy were to improve the validity and reliability of the coding and to assess and express remaining uncertainty.
Data Sources and Type of Coding Tailored to Concepts
No one way of gathering data—through automated text analysis, in-house coding, or expert surveys, to mention three examples—is superior to all others regardless of what type of concept one is trying to measure. Different data collection methods come with different strengths and weaknesses and are thus suitable for different purposes (Skaaning Reference Skaaning2018). The same goes for different data sources. If one wants to collect data on education laws, legal texts are a great source. Suppose one wants to collect data on how education is practiced in the classroom. In that case, legal texts may not represent these practices well, and other sources may be better suited (e.g., classroom observation, secondary sources on education systems, expert surveys, and surveys administered among local nonexperts). Generally, data collection practices and data sources should be tailored to the concept one is trying to measure. Our three education datasets illustrate this point.
EPSM and HEQ set out to code (mainly) de jure characteristics of education systems, whereas V-Indoc explicitly aims to code mainly de facto characteristics that reflect how education is practiced. Coding how complex systems—be it education systems, state bureaucracies, or political regimes—actually work requires considerable in-depth knowledge. Acquiring such case-specific knowledge may be extremely time consuming and thus infeasible for any single researcher or research assistant coding numerous cases. Structured (country-topic) expert surveys are therefore often one effective and appropriate method for collecting and codifying extensive cross-country information in a comparable manner when questions require in-depth case knowledge; answering such questions is presumably less time consuming for experts on a particular country since they can draw on prior knowledge (or already know which references to consult). The ambition to code how education systems work in practice is thus a key rationale behind V-Indoc employing country experts for their coding.Footnote 13
Yet building datasets based on answers from hundreds of experts comes at a cost. Even presenting specific questions, defining key concepts in detail, and ensuring entirely consistent coding is difficult (though, as discussed, using measurement-modeling approaches help). Limited communication between experts and the survey team, as well as between experts, means that implicit, individual coding heuristics may remain (instead of becoming collective and explicit via joint discussions), and divergent interpretations of concepts are hard to catch and clarify. Thus, for data types and concepts that do not require the same amount of in-depth contextual knowledge, it may be preferable to use the same group of (in-house) coders to ensure consistent coding, especially when terms carry multiple meanings (e.g., “primary education level” or “ideological training”). Put differently, the relative benefits of in-house coding compared to expert coding increase when the level of country expertise and contextual knowledge required is smaller and conceptual ambiguity is larger. Sometimes, the sources that must be used also require specific expertise that is not country- but source-specific (e.g., some type of database or a particular type of legal text). In this case, it makes more sense to train a few coders (e.g., research assistants) to code each country than ask experts to do so.
It is possible to devise strategies that harness benefits from different approaches. The HEQ dataset, for example, relies on country-specific expert historians’ local knowledge of the legal educational landscape. This knowledge increases the accuracy and completeness of information about de jure policies, but it also relies on an in-house quality-assurance manager to ensure comparability across countries and consistency in responses within the same country over time. Even for data coded entirely in-house, data creators can develop protocols for finding and checking with people who have knowledge of the local context to obtain information when particular cases have complexities.
Advice for Dataset Users
This section turns to potential pitfalls and advice for users of datasets, focusing on how different dataset characteristics—and similarly sounding variables that differ in subtle ways across datasets—may affect inferences. Specifically, we highlight three issues, where measures across different datasets might capture the same concept, but dataset creators (1) focus on different dimensions of that concept, (2) emphasize de jure or de facto dimensions of this concept, or (3) apply different thresholds when creating categories for the measures. To facilitate comparisons, we harmonize indicators across the three datasets so that they follow a common scale.Footnote 14 We refer to table 2 for a summary and online appendix D for further specific guidelines.
Before we turn to these specific issues, we want to stress a general point about the importance for dataset users to be aware of the features of an existing dataset—including the goals of the dataset creators in collecting the data in the first place—to understand what that dataset can be used for, and what kinds of analyses should be avoided. Suppose that a researcher wants to identify when governments first began to mandate the inclusion of civic education in the curriculum. Both EPSM and HEQ could be used to gain some insight into this question because they contain data on curriculum policies. However, because they (intentionally) focus on national policies, researchers would need to complement what they can learn from these datasets with information about subnational policies obtained from other sources. Alternatively, suppose that a researcher wants to draw general conclusions about education systems globally. In that case, they should opt for datasets like EPSM and V-Indoc, which have good geographic coverage across all regions, and avoid relying on HEQ, which focuses on Europe and Latin America. Finally, suppose a researcher wants to conduct a single-country case study or focused comparisons of education policies pertaining to indoctrination. In that case, they should opt for datasets like HEQ that provide more fine-grained information about each country in the dataset, instead of relying on broader-coverage datasets like EPSM and V-Indoc, which contain data that are suited for analyzing aggregate trends.
Same Name, Different Content
As previewed in the introduction, several dataset creators occasionally attempt to capture the same concept but differ on which dimension of that concept they (want to) measure. The introductory example we used was a multidimensional concept of democracy. Some datasets measure only the presence of contested elections (Cheibub, Gandhi, and Vreeland Reference Cheibub, Gandhi and Vreeland2010), while others incorporate suffrage rights (Boix, Miller, and Rosato Reference Boix, Miller and Rosato2013) or try to measure additional dimensions of democracy such as respect for freedom of speech or other civil rights (Coppedge et al. Reference Coppedge, Gerring, Knutsen, Lindberg, Teorell, Altman and Bernhard2023).
Key concepts in the education literature experience a similar issue, which this section illustrates for the concept of education centralization. Following the emerging literature on education and state building (e.g., Paglayan Reference Paglayan2022a; Reference Paglayan, Jenkins and Rubin2022b), education centralization refers to the concentration of authority over education policy decisions in the hands of the national government (Ansell and Lindvall Reference Ansell and Lindvall2020; Del Río, Knutsen, and Lutscher Reference Del Río, Knutsen and Lutscher2024; Neundorf et al. Reference Neundorf, Nazrullaeva, Northmore-Ball, Tertytchnaya and Kim2024; Paglayan Reference Paglayan2021). High levels of centralization denote that the national government has total control over education, while low levels reflect that education decisions are made at the regional, local, or school level.
While education centralization, as a concept, subsumes all kinds of education policy decisions, most available measures focus on the distribution of authority across government levels for a few policy areas. For example, most studies about education decentralization in Latin America during the 1990s refer specifically to decentralization in the responsibility to fund schools and/or manage their day-to-day operations (Grindle Reference Grindle2004; Kaufman and Nelson Reference Kaufman and Nelson2004; Murillo Reference Murillo1999). In another example, Ansell and Lindvall’s (Reference Ansell and Lindvall2020) binary measure of education centralization is based on who has authority over the appointment, promotion, and payment of teachers. Some case studies instead focus on the presence of national examinations and grading standards (Clarke, Timperley, and Hattie Reference Clarke, Timperley and Hattie2003; Zhao Reference Zhao2012), or national school inspection systems (Cermeño, Enflo, and Lindvall Reference Cermeño, Enflo and Lindvall2022).
EPSM, V-Indoc, and HEQ all offer indices that measure education centralization but emphasize different dimensions (see online appendix B for a summary of how these indices are constructed). EPSM focuses on the (de jure) existence of a national curriculum and includes an additional dimension of national government control over school funding and management (at different levels of education). V-Indoc focuses on national government control over education content by establishing national curricula and approving textbooks. HEQ also measures whether a centralized curriculum exists and whether the national government approves textbooks.Footnote 15 Figure 2 depicts trends in education centralization across the three datasets in the five countries for which our data overlap, first for the comprehensive indices (panel A) and second for the centralization of the curriculum indicators, which are a part of all the indices and have been harmonized to make their scales comparable (panel B). The darker the cells, the more centralized the education system is.
Figure 2 Education/Curriculum Centralization (EPSM, V-Indoc, and HEQ)
Note: See online appendix B for a description of the centralization indices across the three datasets. The EPSM and V-Indoc datasets have missing values for Germany between 1945 and 1949 as both datasets follow V-Dem’s coding of country-years.
We can draw several takeaway points from the figure. First, differences in the dimensions included in education centralization can have important consequences for scores (and thus, e.g., trends) in combined indices. This difference is indicated by comparing the EPSM and HEQ indices in panel A, especially for Chile and Argentina. The diverging index scores suggest that a national government’s control over education content, which is covered by both EPSM and HEQ and displays similar trends across datasets (compare panel B), does not entail that it also controls other aspects of education systems, such as funding and management (only included in the EPSM index). For example, Augusto Pinochet’s Chile (1973–90) maintained a centralized national curriculum but engaged in the decentralization of education funding and management to municipalities (Cox Reference Cox2005).Footnote 16 Indicatively, the major differences between the EPSM and HEQ indices in panel A—which might, at first sight, be interpreted as low reliability for one or both of the measures—mostly disappear when focusing more specifically on curriculum centralization in panel B.
Our first recommendation to dataset users is thus to be aware of the number and type of dimensions covered by the measures that they use. This is especially important when relying on indices, which are commonly used in empirical research and require researchers to be deeply familiar with the complex decisions and indicators involved in the creation of those aggregated measures.
De Jure and de Facto
Another interesting pattern from figure 2 appears when comparing V-Indoc and HEQ. Both measure education centralization based on the curriculum and textbooks but use different data collection methods. This contributes to explaining why these datasets sometimes arrive at different conclusions about the degree of education centralization. Consider the case of Argentina. Between the transition to democracy in 1983 and 1993, Argentina appears to have a more centralized curriculum according to HEQ than according to V-Indoc. The difference is likely to be driven, at least in part, by the fact that V-Indoc experts presumably take into account not only de jure but also de facto centralization, whereas HEQ focuses exclusively on de jure policies.Footnote 17 Indeed, while Argentina’s 1884 law of primary education established a national curriculum for all public schools, its enforcement was imperfect, and in practice subnational governments had leeway to deviate from it, especially after 1983. This informal practice is captured by V-Indoc. HEQ, by contrast, with its focus on de jure policies, only recognizes subnational intervention in the curriculum starting in 1994, when a new law formally recognized the ability of provinces to have some say over the curriculum.Footnote 18
De jure versus de facto distinctions are important in the social sciences. Researchers are, for example, often interested in understanding the extent to which changes in legislation or formal institutions produce changes in policies, practices, or power relations (Acemoglu and Robinson Reference Acemoglu and Robinson2006; Ansell and Lindvall Reference Ansell and Lindvall2020), or whether legislation mostly institutionalizes already existing practices (Paglayan Reference Paglayan2019; Przeworski Reference Przeworski2004). Works on state capacity highlight how and why changes to legislation may not always translate into effective implementation (e.g., Fukuyama Reference Fukuyama2004). Nevertheless, the information that researchers require to empirically study such questions is often unavailable. For researchers interested in understanding education systems, combining datasets such as V-Indoc (mostly de facto), EPSM (mostly de jure), and HEQ (purely de jure) can help to accomplish this goal, as we illustrate in this section.
Several factors can affect gaps between de jure policies and de facto practices, including the state’s fiscal and administrative capacity, the existence of school inspections, political regime type, conflict, or a country’s territorial size (Cermeño, Enflo, and Lindvall Reference Cermeño, Enflo and Lindvall2022; Lopez Reference Lopez2020; Paglayan Reference Paglayan2024). We do not aim to explain what causes those gaps here, which is an important question that we leave for future research. Instead, we use our education data to identify and describe such gaps.
Figure 3 draws on measures from V-Indoc and HEQ to demonstrate the relevance of the de jure versus de facto distinction on one specific dimension of education systems: the politicization of teacher recruitment practices.Footnote 19 HEQ, which focuses on de jure policies, includes measures on whether applicants to teacher education programs must show proof of moral competency (yes/no) or belong to a particular religion (yes/no), and whether public primary-school teachers are required to swear allegiance to the state and/or the constitution (yes/no) or to a particular party or a ruler (yes/no). Using this information, we create a dichotomized indicator of politicization in teacher recruitment that takes a value of zero when neither of these requirements is present and a value of one when at least one is present. In V-Indoc, the indicator of political teacher-hiring measures whether the teacher-hiring criteria are de facto based on teachers’ political views, political behavior, and/or moral character.Footnote 20 The possible answer categories are as follows: rarely or never, sometimes, often, and almost exclusively. To ease comparisons, we dichotomize the V-Indoc indicator: zero means hiring decisions are rarely or never based on politicized criteria, while one combines the three politicized categories (i.e., sometimes, often, and almost exclusively).Footnote 21
Figure 3 Trends in Politicized Teacher Recruitment (V-Indoc and HEQ)
Note: The harmonized indicator for politicized teacher recruitment is coded as one if there are any political or moral requirements to becoming a teacher, and zero otherwise.
When plotting the two measures for five overlapping countries and years in figure 3, we observe both de jure and de facto politicization in teacher recruitment in Germany across the entire period, de jure but not de facto politicization in Italy, and some years of convergence and divergence between the measures in Argentina, Chile, and Spain. Instead of relying only on one dataset measuring either de jure or de facto aspects, contrasting otherwise fairly similar measures from two datasets may give nuanced and important descriptions of the historical developments of education systems.Footnote 22
Let us elaborate on the added informational value of measuring both de jure and de facto aspects by returning to the case of Argentina. In the 1940s and 1950s, the Peronist regime’s administration introduced a requirement for current and new teachers to swear allegiance to the Peronist doctrine as a condition for employment in public schools. The regime purged the profession of numerous teachers—many from a middle-class background—who opposed and refused to swear allegiance to it. This period is aptly captured by both HEQ’s de jure and V-Indoc’s de facto measures of politicization in teacher recruitment. After Juan Perón went into exile in 1955, subsequent national governments removed the legal requirement for teachers to swear allegiance to a specific party or regime and no new legal requirements focused on regulating teachers’ political leanings were introduced, as reflected by the HEQ measure. However, in practice, the politicization of teacher recruitment remained in place for decades. First, members of the Peronist party took control of many teacher-hiring commissions at the subnational level and used that power to favor the appointment of Peronist teachers. Second, during the 1970s, the military dictatorship headed by Rafael Videla persecuted teachers not only of Peronist affiliation but also those suspected of opposing the regime. In other words, as captured by the V-Indoc measure, the politicization of the teaching profession remained in place beyond what the law stipulated during two periods after 1955.
Same Concepts but Different Thresholds
Sometimes, similar measures from different datasets may capture the exact same concept yet use different thresholds to establish coding categories. For instance, measures capturing similar minimalist (electoral) and dichotomous democracy concepts may lead to widely different empirical distributions of regimes if one has a very high bar for considering elections sufficiently “free and fair” and another applies a lower bar (Kasuya and Mori Reference Kasuya and Mori2021). More generally, differences in such thresholds can stem from researchers operating with different (often implicit) theoretical assumptions or even from different data collection strategies. This section illustrates how different thresholds affect inferences by discussing how HEQ and EPSM identify religious education in the curriculum.Footnote 23
Briefly, HEQ aims to codify whether religious education is part of the official curriculum. To do so, it identifies whether religion is included as a compulsory, stand-alone course. The subject need not be about religion exclusively. A subject called “moral and religious education,” for example, would satisfy the HEQ criterion for coding a country as mandating religious education. Similarly, EPSM requires that religious education forms (large parts of) a stand-alone subject in the mandatory curriculum for a country to be classified as having religious education. However, an additional requirement in the case of EPSM is that religion must form part of the current regime’s political system and/or be the basis of an official school of thought that has the status of an “official” ideology in the regime. The latter would be the case, for example, if religion is mentioned in the constitution. This additional criterion restriction reflects EPSM’s aim to capture instruments for indoctrination and regime legitimation (and religion is only one of several relevant “ideology categories” for which compulsory, stand-alone civics courses are coded). As a result of this coding decision, countries with a compulsory, stand-alone religious course where religion is irrelevant to regime ideology will be coded as having religious education in HEQ but not in EPSM. In other words, the added criterion in EPSM means that there is a higher threshold for coding “religious education” in this dataset than in HEQ.
These different thresholds imply that HEQ is more likely to identify religious instruction than EPSM, and this is indeed what we observe in figure 4. One case where the different thresholds help to explain the divergence in how religious instruction is coded across HEQ and EPSM is post-Pinochet Chile. In 1996, Decree No. 40 introduced religion to the curriculum as a compulsory subject, leading the HEQ dataset to identify this change in the curriculum.Footnote 24 However, because religion did not form part of the democratic regime’s ideology after 1996, EPSM does not register this addition of religion into the curriculum.
Figure 4 Religious Instruction in Primary Schools (EPSM and HEQ)
Note: HEQ and EPSM focus on stand-alone compulsory courses to detect religious values in primary education, while V-Indoc examines its presence in history courses. The y-axis reflects a harmonized scale for the three indicators between zero and one. For V-Indoc, the values reflect the proportion of coders (out of the total number of coders) who consider religion to be one of the top two ideologies or dominant models in the history curriculum.
This example of seemingly similar measures carrying different informational content indicates that dataset users should pay careful attention to coding rules and thresholds. This requires spending time reading the fine print in codebooks and other dataset documentation before selecting which measure is most appropriate to use for a particular purpose.
Lessons
This paper has highlighted how various and specific choices on data collection and measurement influence how indicators and indices are scored. We have done so by comparing and contrasting measures from three novel historical datasets on education systems and policies. In addition to detailing various choices faced by dataset creators and their consequences for measurement, we have discussed key challenges and issues that dataset collectors need to be attentive to, as well as strategies for mitigating them. Likewise, we addressed several, and often hard-to-detect, issues that dataset users need to be aware of, specifically highlighting how even measures that may initially seem identical could carry quite different informational content.
We hope our discussions contribute to ongoing debates on measurement—for example, on the appropriateness of relying on expert-coded versus “objective” data—by unveiling limitations in assembling and using datasets of different kinds for different purposes. By demonstrating the importance of even (seemingly) minor assumptions and undercommunicated data collection choices, we also hope that our reflections can promote a shift toward more transparency on data collection process choices and limitations with the resulting datasets. This would, in turn, contribute to enhancing the reliability and replicability of future research.
To help researchers in this endeavor, online appendix D provides a checklist, both for academic data producers and users, based on the lessons we have discussed in this study. We summarize the checklist as a set of guidelines in table 2. One important caveat is that these guidelines reflect our experiences and considerations pertaining to the coding of country-level, historical (education) datasets, and they should not be viewed as the best practices that everyone should follow regardless of the type of data or other considerations (following some of the guidelines for dataset creators does, for example, require substantial resources for coding). Sections D1 and D2 in the online appendix provide more detailed suggestions from each guideline, examples of how to implement them, and discussions of their potential benefits.
For data creators, we invite researchers to apply some of the measures described in this paper (and used for some or all of our three example datasets) to enhance reproducibility and transparency. This entails being explicit about all coding decisions and documenting the data sources underpinning such decisions, especially in tricky cases. If data producers have doubts about coding decisions, they should not be afraid of exposing the limitation but rather explain the source of uncertainty and rationale behind the coding decision (and even plausible, alternative decisions) in the dataset documentation. Besides a detailed codebook, a RoT document could be useful in cases where clear rules are inapplicable or ambiguity in coding decisions remains. Such documentation not only makes coding assumptions explicit to users but also enhances coding consistency by making different coders use the same explicit heuristic instead of several implicit ones.
For data users, a careful reading of articles introducing the dataset, the codebook, and other documentation is a must, as it can reveal key assumptions underlying the dataset and ensure proper interpretation and inference. Datasets are typically based on nontrivial assumptions about the relevant properties that characterize a phenomenon, often linked to research goals. Against this backdrop, dataset users should ensure that they select and cross-check those datasets that match the theoretical assumptions and purposes of their own research.
Depending on the data user’s research design and goals, our study also highlights how one can fruitfully combine variables (also from different datasets) to measure different dimensions of the same concept. Nevertheless, given the caveats noted above, data users should make sure only to use variables that represent appropriate operationalizations of the author’s concept of interest. Our study has highlighted how even variables that seem to be similar and may even have identical names (e.g., “education centralization index”) can tap into very different (dimensions of) concepts, leading to low correlation. When this is the case, “robustness tests” that blindly substitute one variable for another may lead to very different results. Thus, providing a detailed appendix where researchers test whether the results are robust to alternative popular measures that, on the surface, seem similar may lead researchers astray. Instead, we hope that our advice on gathering detailed information and carefully evaluating the relevance of measures could motivate theory-driven discussions of the relevance of particular tests, robustness, and generalization rather than (only) data-driven discussions.
Finally, an implication of our discussions is that medium or low correlations between measures (especially between different datasets) pertaining to the same concept are not necessarily indicative of low reliability in any of the measures assessed. Instead of prematurely concluding that divergences stem from measurement error, data users should closely inspect codebooks, documentation, and descriptions of the different measures, as it is possible that the measures differ because they capture different dimensions of a concept or even different concepts being referred to with the same term. Dataset producers, too, should pay careful attention to be clear and upfront about what concept(s) they measure and how, and they should make comprehensive documentation readily available for users.
Supplementary material
To view supplementary material for this article, please visit http://doi.org/10.1017/S1537592725103058.
Data replication
Data replication sets are available in Harvard Dataverse at: https://doi.org/10.7910/DVN/FU0U8V
Acknowledgments
We are thankful for the input of editors and reviewers, whose contributions improved this paper. Special thanks to Marcus Österman and Sophie Mainz for suggesting to write additional detailed guidelines for data creators as well as to Jane Gingrich, Svend-Erik Skaaning, and Susanne Garritzmann for their critical feedback on an early version of this manuscript. We also appreciate the input of the participants of the 2024 European Political Science Association conference and the Practices of 2024 Comparative-Historical Analysis Conference at Aarhus University. The HEQ database has been funded by numerous centers and programs at Stanford University, including the King Center on Global Development, the Europe Center, Stanford SEED, and the Vice Provost for Graduate Education. V-Indoc was generously funded by the European Research Council Consolidator Grant “Democracy under Threat: How Education Can Save It” (DEMED) (grant no. 865305). EPSM has received funding from the European Research Council under the European Union’s Horizon 2020 research and innovation program (grant no. 863486).
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