6 Sep 00:53 2007

## Specifying random effects for multiple covariates via lmer

While working through the text "Applied Longitudinal Analysis" by Fitzmaurice, Laird and Ware, I encountered a fairly simple case study (pp 210-7) in which a longitudinal model specifies three random effects: (1) random intercepts for id, (2) random slopes for covariate1 (Age | id), and (3) random slopes for covariate2 (log(ht) | id). I've had no difficulty formulating lmer models with correlated random intercepts and slopes for either of the covariates individually but have not succeeded when I try to compose a model with correlated random intercepts and slopes for two covariates. Following is code that works well with the individual covariates separately: m1=lmer(LFEV1~Age + loght + InitAge + logbht + (1 + Age | id),data=fev, na.action=na.omit, method="REML") m2=lmer(LFEV1~Age + loght + InitAge + logbht+(1 + loght | id),data=fev, na.action=na.omit, method="REML") I have yet to write code that succeeds with both covariates simultaneously. I am running V 2.5.1 of R and the most recent version of lme4. Any advice will be appreciated. If more detail is needed, I can send it but thought this might be sufficient to pose the problem. [[alternative HTML version deleted]]