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Vance, Colin
Ritter, Nolan
Year of Publication: 
Series/Report no.: 
Ruhr Economic Papers 466
Sample selection models, variants of which are the Heckman and Heckit models, are increasingly used by political scientists to accommodate data in which censoring of the dependent variable raises concerns of sample selectivity bias. Beyond demonstrating several pitfalls in the calculation of marginal effects and associated levels of statistical significance derived from these models, we argue that many of the empirical questions addressed by political scientists would – for both substantive and statistical reasons – be more appropriately addressed using an alternative but closely related procedure referred to as the two-part model (2PM). Aside from being simple to estimate, one key advantage of the 2PM is its less onerous identification requirements. Specifically, the model does not require the specification of so-called exclusion restrictions, variables that are included in the selection equation of the Heckit model but omitted from the outcome equation. Moreover, we argue that the interpretation of the marginal effects from the 2PM, which are in terms of actual outcomes, are more appropriate for the questions typically addressed by political scientists than the potential outcomes ascribed to the Heckit results. Drawing on data compiled by Sweeney (2003) from the Correlates of War database, we present an empirical analysis of conflict intensity illustrating that the choice between the sample selection model and 2PM can bear fundamentally on the conclusions drawn.
Abstract (Translated): 
Stichproben-Modelle, wie beispielsweise Heckman- und Heckit-Modelle, werden zunehmend von Politikwissenschaftlern benutzt. Sie bergen jedoch gewisse Probleme. Viele der Fragen, die von empirischen Politikwissenschaftler untersucht werden, könnte man - sowohl aus inhaltlichen als auch aus statistischen Gründen - besser mit einem alternativen, wenn auch eng verwandten Verfahren wie dem 'two-part model (2PM)' analysieren. Abgesehen davon, dass es einfach zu schätzen ist, ist eines seiner Vorteile, dass die Identifikations-Anforderungen geringer sind. Gestützt auf Daten, die von Sweeney (2003 ) aus der 'Correlates of War'-Datenbank zusammengestellt worden sind, präsentiert das Papier eine empirische Analyse der Konfliktintensität, die zeigt, dass die Wahl zwischen einem herkömmlichen Stichproben-Modell und das 'two-part model' fundamentale Auswirkungen auf die gezogenen Schlussfolgerungen haben kann.
Heckit model
two-part model
potential effects
actual effects
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Document Type: 
Working Paper

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