Re: st: Panel data - XTGEE with 2 levels of clustering

[Paul Millar <paul.millar@shaw.ca>]

Hi Luciana,

I don't think this is a case for using panel regression: there is no 
dimension of time, since the order of the visits doesn't matter.
However, you must allow for the fact that the measures on a given 
subject are not independent.
Since the data are not distributed normally, you might use a gamma model.
If the eye doesn't matter either, I would suggest a regular model 
clustered on subject:

glm slabpeak wdtpeak,family(gamma) cluster(subj)
pre

The above handles repeated measures by clustering on subject 
(relaxing the assumption of independence).

- Paul




At 08:07 PM 01/04/2009, you wrote:
> Dear Statalist,
>
> I would like to ask some help on a multilevel unbalanced model 
> (subjects - eyes - visits).
> I have several subjects (subj). For most subjects both eyes were 
> included (eye), and each eye was assessed from 1 to 3 different 
> times (visits) under different medications (the order of the visits 
> is not important). Intraocular pressure (IOP - continuous variable) 
> was measured by two different methods in each occasion (iop_slab and iop_wdt).
> I am interested in the comparison between these two methods (more 
> specifically if wdt_iop can "predict" slab_iop), but I need to 
> adjust for the repeated measures in each eye and for the correlation 
> between both eyes of a same patient.
>
> I've found some examples and publications online, but mostly with 
> only two levels. I am relatively new to Stata. I'm using Stata/MP 10 
> on Windows. I believe the best option is to use xtgee, I'm only not 
> sure how to include both levels.
>
> I've pasted a sample of my data below:
>
> subj eye visit wdtpeak slabpeak
> 22016 OD 1 25.50 18.50
> 22016 OS 1 19.50 19.50
> 22019 OD 1 16.50 22.50
> 22019 OS 1 15.25 21.00
> 22020 OD 1 23.00 22.00
> 22020 OD 2 20.00 20.00
> 22020 OS 1 23.50 23.00
> 22020 OS 2 18.00 20.50
> 22021 OD 1 33.00 34.50
> 22021 OD 2 31.00 30.50
> 22021 OS 1 26.50 30.00
> 22021 OS 2 25.00 27.50
> 22022 OD 1 18.00 23.00
> 22022 OD 2 16.00 23.00
> 22022 OS 1 20.00 25.50
> 22022 OS 2 17.50 23.00
> 22023 OD 1 21.50 22.50
> 22023 OD 2 21.50 23.50
> 22023 OS 1 20.00 22.00
> 22023 OS 2 20.00 23.00
> 22024 OD 1 22.00 22.00
> 22024 OD 2 19.00 22.50
> 22024 OD 3 18.00 23.00
>
> Thank you very much.
>
> Luciana
>
>
>
> Luciana M. Alencar, M.D.
> University of California , San Diego
> Hamilton Glaucoma Center room 174
> 9500 Gilman Drive, Dept. 0946
> La Jolla, CA 92093-0946
> phone: 858.534.5334
> fax: 858.822.0615
> email: lalencar@glaucoma.ucsd.edu<mailto:atafreshi@glaucoma.ucsd.edu>
>
>
>
>
> *
> *   For searches and help try:
> *   http://www.stata.com/help.cgi?search
> *   http://www.stata.com/support/statalist/faq
> *   http://www.ats.ucla.edu/stat/stata/

*
*   For searches and help try:
*   http://www.stata.com/help.cgi?search
*   http://www.stata.com/support/statalist/faq
*   http://www.ats.ucla.edu/stat/stata/