longitudinal mixed model troubles

jax

New Member
#1
I have growth data for 5 time points. In my unconditional model the quadratic trend best fits the data (likelihood ratio test). However, the random effect for the quadratic term is not significant but the fixed effect is significant. I also have trouble when adding covariates - the g matrix is no longer positive-definite. Does it make sense to remove the quadratic random effect but leave the fixed effect in the model? Or just estimate a linear trend instead?
 

spunky

Can't make spagetti
#2
i think it does make sense. you tested it, you saw it is constant across time points so you can leave it in.

or is this covariate the one that's generating the non-positive definitness? (that's usually ugly, ugly stuff)