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Measuring Subgroup Preferences in Conjoint Experiments

Published online by Cambridge University Press:  07 August 2019

Thomas J. Leeper*
Affiliation:
Department of Methodology, London School of Economics and Political Science, London WC2A 2AE, UK. Email: thosjleeper@gmail.com
Sara B. Hobolt
Affiliation:
Department of Government, London School of Economics and Political Science, London WC2A 2AE, UK. Email: s.b.hobolt@lse.ac.uk
James Tilley
Affiliation:
Department of Politics and International Relations, University of Oxford, Oxford OX1 3UQ, UK. Email: james.tilley@politics.ox.ac.uk

Abstract

Conjoint analysis is a common tool for studying political preferences. The method disentangles patterns in respondents’ favorability toward complex, multidimensional objects, such as candidates or policies. Most conjoints rely upon a fully randomized design to generate average marginal component effects (AMCEs). They measure the degree to which a given value of a conjoint profile feature increases, or decreases, respondents’ support for the overall profile relative to a baseline, averaging across all respondents and other features. While the AMCE has a clear causal interpretation (about the effect of features), most published conjoint analyses also use AMCEs to describe levels of favorability. This often means comparing AMCEs among respondent subgroups. We show that using conditional AMCEs to describe the degree of subgroup agreement can be misleading as regression interactions are sensitive to the reference category used in the analysis. This leads to inferences about subgroup differences in preferences that have arbitrary sign, size, and significance. We demonstrate the problem using examples drawn from published articles and provide suggestions for improved reporting and interpretation using marginal means and an omnibus F-test. Given the accelerating use of these designs in political science, we offer advice for best practice in analysis and presentation of results.

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Copyright
Copyright © The Author(s) 2019. Published by Cambridge University Press on behalf of the Society for Political Methodology.

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