** Linear Poisson model;
PROC GENMOD DATA=aids;
  MODEL Cases = Year / DIST=POI;
RUN;

** Null model;
PROC GENMOD DATA=aids;
  MODEL Cases = / DIST=POI;
RUN;

** Rate ratios;
PROC GENMOD DATA=aids;
  MODEL Cases = Year / DIST=POI;
  ESTIMATE '1-year' Year 1 / E;
  ESTIMATE '3.5-year' Year 3.5 / E;
  ESTIMATE '5-year' Year 5 / E;
RUN;

** Obtain likelihood ratio intervals and tests;
PROC GENMOD DATA=aids;
  MODEL Cases = Year / DIST=POI LRCI TYPE3;
RUN;

** Residual/diagnostic plots;
PROC GENMOD DATA=aids PLOTS=ALL;
  MODEL Cases = Year / DIST=POI LRCI TYPE3;
RUN;