Yes, unfortunately the pilot data have only 2 patients. Many thanks for
your helpful advice.

Cheers,
Angelina

On Thu, May 3, 2012 at 1:31 PM, Ben Bolker <bbolker@gmail.com> wrote:

> Angelina Mukherjee <angelina.mukherjee88@...> writes:
>
> > I have response measures corresponding to 2 patients. The structure is as
> > follows:
> >
> > Patient 1:  Region 1             Region 2              Region 3
> >                S1 S2 S3 S4       S1 S2 S3 S4        S1 S2 S3 S4
> >
> > Patient 2:  Region 1               Region 2            Region 3
> >                 S1 S2 S3 S4       S1 S2 S3 S4        S1 S2 S3 S4
>
>
>   Hmm.  Do you really have only two patients, i.e. a total of
> 24 response values?  I understand that you're trying to do a
> variance decomposition here (no fixed effects, only random
> effects), but your estimates of variance will be extremely
> inaccurate based on only two patients (you might want to consider
> making patient a fixed effect, then you would at least have
> 6 data points (5 df) for the patient:region variance ...
>
> > Each patient has 3 regions and each region has 4 sub-regions. (nested
> > design)
> >
> > Fitting   *lme( Response ~ 1, random=~ Patient + Region +Subregion |
> > Patient/Region/Subregion )*
> > allows me to specify covariance structure for the 'sub-region' term.
> >
> > But I'm trying to fit a random effects model of the form as I have only 1
> > observation per 'sub-region':
> > *lme( Response ~ 1, random=~ Patient + Region | Patient/Region )*
> >
> > Is there a way I can specify a covariance structure like
> > the auto-regressive (to specify that correlation decreases with distance
> as
> > one moves from Subregion 1 to Subregion 4) for the 'sub-region' term only
> > as it is not included in my random effects model but I'd like to account
> > for the correlation in it?
>
>    I would think that something like
>
> lme(Response~1, random = ~1|Patient/Region,
>   correlation=corAR1(~Subregion))
>
>  But I also think you're fitting a more complicated model
> than can really be supported by 24 data points ...
>
>  Ben Bolker
>
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