Hi, I have a question regarding this topic. This was a good explanation on this topic, but I'm still confused. I have matched retrospective cohort data. The data was retrospective, meaning that the "cases" occurred in the past and are in a database (no follow up). An epidemiologist decided to match exposures to non-exposures (always 1 : 5) and make this a retrospective cohort. I used PROC PRINT to count pairs, populate a matched 2x2 table, and caculate risk ratios. Then, I confirmed my results with PHREG using a strata statement (DNBI = case (0,1), DNBI12 = case (2,1) waiver (0,1) = exposure, ID = each matched pair of 6 (1 exposure, 5 non-exposures)): proc phreg data=comb nosummary; strata id; class waiver sex; model dnbi12*dnbi(0) = waiver /ties=breslow risklimits ; run; I get the same results with and without the strata statement. Now, exposures were matched to unexposed on sex. So, when I throw sex into the model and code above model dnbi12*dnbi(0) = waiver sex /ties=breslow risklimits ; I get no answer for the effect of sex on dnbi. However, with the strata statement removed (ignoring the strata variable), I get a signficant result for sex. I found this on the web: "If matching was done appropriately, and matching is not taken into account in the analysis, the OR will be biased towards the null." Therefore, I interpret this as, if I ignore the strata variable, the OR (or hazard, in this case) will be biased towards 1.0. If it is still significant, then we can be assured of a signficant effect of sex on dnbi, but the effect is biased and lower than the true effect. Is this an appropriate interpretation?
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