st: Risk differences in Epitab

[Edward Norton <enorton@email.unc.edu>]

Statalist,
   I would like to compute a risk difference, instead of the default 
risk ratio, using the -cs- command in epitab.  However, when I try to 
run commands like

gen ones = 1
cs y x, by(a b c) standard(ones) rd

Stata 9.0 returns a error of "invalid sytax" number 198.  The same 
command without "rd" (meaning the default risk ratio) works fine.
   Any suggestions about how to compute risk differences would be 
appreciated.
   Edward
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