Another case of -1.0 correlation of random effects

Kevin E. Thorpe <kevin.thorpe at ...> writes:

My data come from a crossover trial and are balanced.

 > str(gluc)

'data.frame':	96 obs. of  4 variables:
  $ Subject  : int  1 2 3 5 6 7 10 11 12 13 ...
  $ Treatment: Factor w/ 2 levels "Barley","Oat": 1 1 1 1 1 1 1 1 1 1 ...
  $ Dose     : int  8 8 8 8 8 8 8 8 8 8 ...
  $ iAUC     : num  110 256 129 207 244 ...

clip>

Shouldn't you make Subject into a factor?

Ken

It would make the plot a little bit prettier but I don't think it
matters in this case because variable that appears as a grouping
variable (i.e. on the right of the | ) is automatically treated as a
factor?  I think?

  Since it is really a crossover trial, it would seem reasonable in
principle to have the (Treatment|Subject) random effect in there as
well. I'm not sure what to do about the -1 correlation: it seems the
choices (not necessarily in order) are (1) throw up your hands and say
there's not enough data to estimate independently; (2) try WinBUGS,
possibly with slightly informative priors; (3) try using lme4a to create
profiles of the parameters and see if you can figure out what's happening.

Ben Bolker
Associate professor, Biology Dep't, Univ. of Florida
bolker at ufl.edu / people.biology.ufl.edu/bolker
GPG key: people.biology.ufl.edu/bolker/benbolker-publickey.asc