Hello, I am working on difference in difference analysis for longitudinal data. The goal is to investigate the difference on the health care cost by comparing intervention group and control group. We collect response variable at three timepoints: baseline, first year and second year. I know that hypothesis testing for D-I-D can be specify as following if there are only pre and post (baseline and first year) involved. My question is how I should specify hypothesis testing for D-I-D if one more year data was added. Your help is greatly appreciated. proc GENMOD data= data_set; class id treatment(ref='0') post(ref='0'); model cost =treatment post treatment*post / dist=gamma link=log type3; repeated subject=id / type=un; estimate "DID Post-Pre" treatment*post 1 -1 -1 1; lsmestimate treatment*post "DID Post-Pre" 1 -1 -1 1; run;
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