Using Margins to test for group differences in growth trajectories in generalized linear mixed models

Sarah Mustillo (), Landerman Lr and Land Kl
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Sarah Mustillo: Purdue University, Department of Sociology
Landerman Lr: Duke University School of Medicine
Land Kl: Duke University Department of Sociology

CHI11 Stata Conference from Stata Users Group

Abstract: To test for group differences in growth trajectories in mixed (fixed and random effects) models, researchers frequently interpret the coefficient of group-by-time product terms. While this practice is straightforward in linear mixed models, testing for group differences in generalized linear mixed models is more complex. Using both an empirical example and simulated data, we show that the coefficient of group-by-time product terms in mixed Logistic and Poisson models estimate the multiplicative change with respect the baseline rates, while researchers often are more interested in differences in the predicted rate of change between groups. The latter can be obtained by using the -margins- command in Stata. This may be especially desirable when the mean of the outcome variable is low and marginal change differs from multiplicative change. We propose and illustrate the use of -margins- to interpret group differences in rate of change over time following estimation with generalized linear models.

Date: 2011-07-20
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