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

[Kevin E. Thorpe]

Ben Bolker wrote:
> 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.

Let's see.  I wish (1) was an option.  (2) would be promising if my 
knowledge of BUGS and Bayesian methods filled more than a thimble. 
Thanks to Jarrod for his suggestion in response to this.  I'll take a 
look at that too.  Option (3) is probably worth a go too.

Aside from the fact that the Dose variable are the actual doses and not 
categories, and we all know not to categorize continuous variables, what 
are your thoughts on treating Dose as a factor (since it seems to behave)?

Thanks all for taking the time to provide your suggestions.

Kevin

-- 
Kevin E. Thorpe
Biostatistician/Trialist, Knowledge Translation Program
Assistant Professor, Dalla Lana School of Public Health
University of Toronto
email: kevin.thorpe at utoronto.ca  Tel: 416.864.5776  Fax: 416.864.3016