lme: predictions variance collapses when one more level is added

Ben Bolker · 2013-10-25T22:48:23Z

Dieter Menne writes: > > I have a simple mixed-model, with predictive factor treat (levels M1,M2,M3, > M4), continuous par, and a grouping variable subj from a cross-over > experiment. > > Everything works as expected when I only use M1, M2, M3; see subset.lme > below. The residuals are well distributed; > resid(.,type="p")~fitted(.)|treat > > When I add level M4 (all.lme below), the variance of the > predictions shrinks > to almost zero. I know that level M4 adds he

Ben Bolker

Fri, Oct 25, 2013 3:48 PM

Dieter Menne <dieter.menne at ...> writes:

Sorry for snipping context here (I'm posting via gmane, which doesn't
like that).  If I use  weights=varIdent(form=~1|treat)) rather than
weights=varPower() (i.e. residual variance varies by treatment group,
rather than as a power function of the estimated mean), I get what
seem (at least at a quick glance) to be reasonable results.

lme: predictions variance collapses when one more level is added

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