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Beyond the Hazard Ratio: Generating Expected Durations from the Cox Proportional Hazards Model

  • Jonathan Kropko and Jeffrey J. Harden
Abstract

The Cox proportional hazards model is a commonly used method for duration analysis in political science. Typical quantities of interest used to communicate results come from the hazard function (for example, hazard ratios or percentage changes in the hazard rate). These quantities are substantively vague, difficult for many audiences to understand and incongruent with researchers’ substantive focus on duration. We propose methods for computing expected durations and marginal changes in duration for a specified change in a covariate from the Cox model. These duration-based quantities closely match researchers’ theoretical interests and are easily understood by most readers. We demonstrate the substantive improvements in interpretation of Cox model results afforded by the methods with reanalyses of articles from three subfields of political science.

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Department of Politics, University of Virginia (email: jkropko@virginia.edu); Department of Political Science, University of Notre Dame, (email: jeff.harden@nd.edu). Previous versions of this article benefited from presentation at the 2014 Annual Meeting of the Society for Political Methodology, Athens, GA. We would also like to thank Justin Kirkland, Bruce Desmarais, Meg Shannon, Fred Boehmke, Anand Sokhey, the editor and three anonymous reviewers for helpful comments. Data replication sets and code are available in Harvard Dataverse at: https://dx.doi.org/10.7910/DVN/ELT9VD and online appendices at https://doi.org/doi: 10.1017/S000712341700045X.

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British Journal of Political Science
  • ISSN: 0007-1234
  • EISSN: 1469-2112
  • URL: /core/journals/british-journal-of-political-science
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