[R-sig-ME] Another case of -1.0 correlation of random effects

[Ken Knoblauch]

Yes. It was a knee-jerk reaction, recalling a trap that
I fell into before with glm.  But, lmer seems to handle
this smoothly for the grouping factor.  Thanks.

Ken


Quoting Ben Bolker <bolker at ufl.edu>:

> Ken Knoblauch wrote:
>> 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
>



-- 
Ken Knoblauch
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