something like that should work :

aggregate(test, list(test[,1]), mean)


2008/8/1 Bertolt Meyer <bmeyer@sozpsy.uzh.ch>

> Dear R users,
>
> I have a newbie-question that I couldn't resolve after reading through
> several pieces of documentation and searching the archive.
>
> I have a data.frame containing experimental data from a group experiment in
> psychology. Each line represents a single participant, but participants were
> assigned to groups of three or four persons. One variable indicates each
> participants' group number (groupID). For a large number of variables, I
> would like to obtain the mean group value. I figured I use tapply() in the
> fashion of tapply(variable, groupID, mean), but that would be a tiresome
> task for my 150 variables. I am thus looking for a way to obtain a
> data.frame that contains one row for each group with the group-mean
> variables as columns.
>
> Example:
>
> > test <- as.data.frame(cbind(c(rep(1,5),rep(2,5)), rnorm(10), rnorm(10)))
> > names(test)[1] <- "groupID"
> > test
>
>   groupID          V2          V3
> 1        1 -0.82990860 -0.61778919
> 2        1 -0.01379452  0.64609053
> 3        1 -2.64990839 -1.00570627
> 4        1 -0.07903878 -0.70864441
> 5        1  0.61483071 -1.32039565
> 6        2 -0.18913937  1.38490710
> 7        2 -0.60017953  0.15893421
> 8        2 -0.99901931  0.05963436
> 9        2 -1.46759515  0.35040283
> 10       2 -0.44650422 -0.08713162
>
> > tapply(test$V2, test$groupID, mean)
>         1          2
> -0.5915639 -0.7404875
>
> > tapply(test$V3, test$groupID, mean)
>         1          2
> -0.6012890  0.3733494
>
> I am now looking for something that gives me
>
>  groupID         V2            V3
> 1       1 -0.5915639    -0.6012890
> 2       2 -0.7404875    0.3733494
>
> Any ideas?
>
> Thank you very much,
> Bertolt
>
> --
> Bertolt Meyer
> Oberassistent
> Sozialpsychologie, Psychologisches Institut der Universität Zürich
> Binzmühlestr. 14, Box 15
> CH-8050 Zürich
>
> bmeyer@sozpsy.uzh.ch
> tel:   +41446357282
> fax:   +41446357279
> mob:   +41788966111
>
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>

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