I wonder why the odds ratio estimated by proc logistic differs from the one obtained from proc freq and proc genmod in the example below (SAS 9.4 TS Level 1M5). The OR's become identical when the low frequency event=0/factor=1 is set to above 20. Using logistic regression in STATA on this data gives the same OR as the one from proc freq and proc genmod. I would like to know why proc logistic gives a slightly different OR (I know exact logistic regression should be used on sparse data).    data check; do i=1 to 52; event=1; factor=1; output; end; do i=1 to 287; event=1; factor=0; output; end; do i=1 to 1; event=0; factor=1; output; end; do i=1 to 385; event=0; factor=0; output; end; run; proc logistic data=check descending; class factor/param=ref ref=first; model event=factor; run; proc freq data=check; tables event*factor/cmh; run; ... View more