hello,
i want to compute the mean of a variable ("aps") for every class
(1,2, and 3).
every id have a few obs., "aps" and class are constant over id.
like this:
id   aps     class
1      11       2
1      11       2
1      11       2
1      11       2
1      11       2
2       8        3
2       8        3
2       8        3
3      12       2
3      12       2
.
.

i tried:

tapply(icu1$aps_st, icu1$hidclass, function(z) mean(unique(z)))

but it's counting every row and not every id.

thank you,

Sigalit.

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