Using Margins to test for group differences in growth trajectories in generalized linear mixed models
AbstractTo 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.
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Bibliographic InfoPaper provided by Stata Users Group in its series CHI11 Stata Conference with number 8.
Date of creation: 20 Jul 2011
Date of revision:
This paper has been announced in the following NEP Reports:
- NEP-ALL-2011-08-09 (All new papers)
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