I want to model two different time variables, some of which are heavily collinear in my data (age + cohort = period). Doing this I ran into some trouble with
lmer and and interactions of
poly(), but it's probably not limited to
lmer, I got the same results with
Obviously, my understanding of what the poly() function does is lacking. I understand what
poly(x,d,raw=T) does and I thought without
raw=T it makes orthogonal polynomials (I can't say I really understand what that means), which makes fitting easier, but doesn't let you interpret the coefficients directly.
I read that because I'm using the predict function, the predictions should be the same.
But they aren't, even when the models converge normally. I'm using centered variables and I first thought that maybe the orthogonal polynomial leads to higher fixed effect correlation with the collinear interaction term, but it seems comparable. I've pasted two model summaries over here.
These plots hopefully illustrate the extent of the difference. I used the predict-function which is only available in the dev. version of lme4 (heard about it here), but the fixed effects are the same in the CRAN version (and they also seem off by themselves, e.g. ~ 5 for the interaction when my DV has a range of 0-4).
The lmer call was
cohort2_age =lmer(churchattendance ~
poly(cohort_c,2,raw=T) * age_c +
ctd_c + dropoutalive + obs_c + (1+ age_c |PERSNR), data=long.kg)
The prediction was fixed effects only, on fake data (all other predictors=0) where I marked the range present in the original data as extrapolation=F.
I can provide more context if need be (I didn't manage to produce a reproducible example easily, but can of course try harder), but I think this is a more basic plea: explain the
poly() function to me, pretty please.