How to use proc logistic to estimate probability difference -- or common risk difference

[HappySASUE]

I try to use proc logistic regression to estimate the probability difference between placebo and treatment group. It gives me odds ratio difference, how can I change to risk difference?   

Also any difference between "risk difference" or "proportional difference"  or "probability difference" between proc freq and proc logistic? 

Thanks, 

[Ksharp · Accepted solution]

Better post it at Statistical Forum ,since it is about stat . @StatDave  @Rick_SAS  is there .

If your data is contingency table , like

           placebo   treatment

good    12           23

bad      2              3

you could try

PROC FREQ;

table group*good_bad / riskdiff ;

run;

"risk difference" is for   12/(23+12)  V.S.  2/(2+3)   or  23/(23+12)  V.S.  3/(2+3) 

"proportional difference"(a.k.a  odds ratio) is for 12/23  V.S.  2/3

[HappySASUE]

Thank you!
Actually I find a very useful link. it explained everything:
http://support.sas.com/kb/37/228.html