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...lesi <- x$SALES[i] med <- median(x$SALES[-i]) print (salesi - med) } } perf(GROUP11) # Why the above funtion doesn't work in the following? tapply(x1$SALES, list(x1$YEAR, x1$GROUP), perf) aggregate(x1$SALES, by=list(x1$YEAR, x1$GROUP),FUN=perf) Thanks! Chingfu Chang