Calculating statistics on subsets of data [duplicate]

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Use the base R function ave(), which despite its confusing name, can calculate a variety of statistics, including the mean:

within(mydata, mean<-ave(measure, subject, FUN=mean))

  subject time measure      mean
1       1    0      10 10.000000
2       1    1      12 10.000000
3       1    2       8 10.000000
4       2    0       7  2.333333
5       2    1       0  2.333333
6       2    2       0  2.333333

Note that I use within just for the sake of shorter code. Here is the equivalent without within():

mydata$mean <- ave(mydata$measure, mydata$subject, FUN=mean)
mydata
  subject time measure      mean
1       1    0      10 10.000000
2       1    1      12 10.000000
3       1    2       8 10.000000
4       2    0       7  2.333333
5       2    1       0  2.333333
6       2    2       0  2.333333

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