We have used Proc Genmod with the ‘repeated’ statement, to estimate in a GEE model the risk difference between the treatment groups for paired proportion. We have used the LINK=IDENTITY option with Binomial distribution to estimate the differences of population probabilities, as the default logit link produces differences in log odds (logits) rather than differences in population probabilities. ods output Diffs=Diff LSMeans=LSMean LSMEstimates=LSMEstimate;
proc genmod data=adqs;
class subjid TRTEXPN;
model response (event='1')=TRTEXPN/dist=binomial link=identity;
repeated subject=subjid/type=un;
lsmeans TRTEXPN/diff cl;
lsmestimate TRTEXPN 'Princess (1) Vs Juvéderm (2)' 1 -1/cl upper testvalue=-0.1 alpha=0.025;
run; We received below question from Regulatory. In order to answer to regulatory could you please share with me the mathematical formula for the variance of the difference in estimate responder rates when using the link=identity? Also could you let us know if there is an alternative approach in SAS to confirm the results from above procedure? For primary analysis, you used a generalized linear model where the response variable “Responder” was assumed to be binomial. In your genmod model, the link function was identity. It is unclear how the SAS genmod procedure solved the model for the binomial response variable using the identity link. Please provide the mathematical formula for the variance of the difference in estimate responder rates (Ptest-Pcontrol), and provide the mathematical formula for the test statistics of the McNemar type test as well.
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