[R] Linear Trend in Residiuals From lme

[Douglas Bates]

On 8/9/06, Rick Bilonick <rab45+ at pitt.edu> wrote:
> I'm fitting a mixed effects model:
>
> fit.1 <- lme(y~x,random=~1|id,data=df)
>
> There are two different observations for each id for both x and y. When
> I use plot(fit.1), there is a strong increasing linear trend in the
> residuals versus the fitted values (with no outliers). This also happens
> if I use random=~x|id. Am I specifying something incorrectly?

Could you provide a reproducible example please?

I suspect that the problem comes from having only two observations per
level of id.  When you have very few observations per group the roles
of the random effect and the per-observation noise term in explaining
the variation become confounded.  However, I can't check if this is
the case without looking at some data and model fits.