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


From   Paul Millar <paul.millar@shaw.ca>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Panel data - XTGEE with 2 levels of clustering
Date   Thu, 02 Apr 2009 05:10:11 -0400

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>




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