Title: Testing for Interaction in Binary Logit and Probit Models: Is a Product Term Essential? Authors: William Berry, Justin Esarey, Jacqueline Rubin Entrydate: 2007-05-06 20:51:39 Keywords: interaction, logit, probit Abstract: Political scientists presenting binary dependent variable (BDV) models often offer hypotheses that independent variables interact in their influence on the probability that an event Y occurs, Pr(Y). A consensus appears to have evolved on how to test such hypotheses: (i) estimate a logit or probit model including product terms to specify the interaction, (ii) test the hypothesis by determining whether the coefficients for these terms are statistically significant, and (iii) if they are, describe the nature of the interaction by estimating how the marginal effect of one independent variable on Pr(Y) varies with the value of the other independent variables. We contend that in the BDV context, statistically significant product term coefficients are neither necessary nor sufficient for concluding that there is substantively meaningful interaction among variables in their influence on Pr(Y). Even when no product terms are included in a logit or probit model, if the marginal effect of one variable on Pr(Y) is related to another independent variable then substantively meaningful interaction is present, and describing such interaction is essential to an accurate portrayal of the data generating process at work. We propose a strategy for studying interaction in the BDV context that is consistent with the recent emphasis in the discipline on casting hypotheses in terms of effects on the probability of an event�s occurrence and reporting estimated marginal effects on this probability. http://polmeth.wustl.edu/retrieve.php?id=692