hi Tasneem
last time I checked, singleton clusters weren't used in the estimation. but I forwarded your question to a friend and colleague John Preisser who wrote the following in reply:
"
First. clusters of size one do not contribute any information to the estimation of within-cluster correlation so the observation Tasneem makes about GENMOD is not specific to that procedure. Second, the first-order GEE (GEE1) of Liang and Zeger does not provide variance estimation for within-cluster association parameter estimates, nor significance testing for them; it uses simple moment estimators. Next, confidence interval estimation of within-cluster correlations would generally be of greater interest than significance testing. If the outcome is binary, the alternating logistic regressions options in GENMOD can be used to estimate some simple structures of within-cluster association based on the pairwise odds ratios. For estimating correlations, Prentice's (1988) GEE procedure is useful for large clusters of correlated binary data. It provides estimates and standard errors for both parameter estimates in the marginal mean model and in the marginal pairwise correlation model. I have developed a SAS/IML macro for Prentice's GEE which can be downloaded from my website. Admittedly, the documentation could be better, but it is easy to use, once the Z matrix for the correlation model is set up (analogous to the X matrix for the marginal mean model).
"I could provide more detail if Tasneem wants to contact me after Wednesday. As I am not a member of this section or forum, please feel free to post my response and adapt it as you see fit. If you wish to attribute the response to me (not necessary), just put the above paragraph as written in quotes." John Preisser
[jpreisse@bios.unc.edu] -------------------------------------------
Stuart Gansky
University of California, San Francisco
-------------------------------------------