how about:
d1=data.frame(pat=c(rep('a',3),'b','c',rep('d',2),rep('e',2),'f'),var=c(1,2,3,1,2,2,3,2,4,4))
ds=set(d1,var %in% c(2,3))
with(ds,tapply(var,pat,FUN=length))
hth,
David Freedman, CDC, Atlanta
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