I have read several of the papers that model non-linear "holiday effects" in longitudinal data with many waves. This makes sense to me conceptually, but I wanted to make sure I understood the proper syntax.
I am 8 waves of drinking data, and there seems to be a large dip in drinking for two months in the midst of winter, meaning the data is not fit well by a linear or quadratic model alone.
If I want to model this drop, is the following syntax correct?
One more quick question related to this -- if I am interested in modeling an effect like this that occurs at only one time point (e.g., just binge7) or at multiple, non-consecutive time points (e.g., binge3 and binge7), what would be the best syntax for that? I've noticed that the intercept only model causes some issues, even if I hold the variance at 0 for winter (and there is a lot of variance in winter when 7 & 8 are included, so the 0 variance model probably isn't the best)...
Perhaps the following paper that is on the website will help:
Greenbaum, P.E., Del Boca, F.K., Darkes, J., Wang, C. & Goldman, M.S. (2005). Variation in the drinking trajectories of freshman college students. Journal of Consulting and Clinical Psychology, 73, 229-238