Dear All, In calculating the relative risk and corresponding exact 95% confidence intervals via exact Poisson regression using a log-linear model, the following scenario works (note that number of cases in group 2 = 1486); data have1; * number of cases in group 2 = 1486 ;
input total cases group all;
log_total = log(total);
datalines;
14660 1529 1 1
14645 1486 2 1
;run;
proc genmod data=have1 exactonly;
CLASS group(ref='1') all /PARAM=ref;
model cases=group all /DIST=poisson OFFSET=log_total LINK=log;
exact group /estimate outdist=dist ALPHA=0.05;
exactoptions statustime=10 method=NETWORK;
ODS OUTPUT ExactParmEst=estimate ExactTests=ExactTest;
RUN; The relative risk, lower and upper confidence limits are got by exponentiating the variables "estimate", "LowerCL" and "UpperCL" in the "estimate" dataset output via ODS OUTPUT. HOWEVER, with a very slight change (number of cases = 1485), a computational issue is encountered, namely, "WARNING: Invalid confidence interval detected and set to missing". data have2; * number of cases in group 2 = 1485 ;
input total cases group all;
log_total = log(total);
datalines;
14660 1529 1 1
14645 1485 2 1
;run;
proc genmod data=have2 exactonly;
CLASS group(ref='1') all /PARAM=ref;
model cases=group all /DIST=poisson OFFSET=log_total LINK=log ;
exact group /estimate outdist=dist ALPHA=0.05;
exactoptions statustime=10 method=NETWORK;
ODS OUTPUT ExactParmEst=estimate ExactTests=ExactTest;
RUN; What could be the issue here? Is this a potential bug in the genmod procedure? How can I solve this? I know a possible work-around is to set the reference group to group(ref='2') which results in the sign of the estimates being flipped. We can back-flip the sign of the estimates and get the needed RR, Lower and Upper limits by exponentiating. But this is just a work-around. What is the solution? Many thanks in advance, George
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