[R] Newbie question: How to use tapply() on several vectors simultaneously

[Stephan Kolassa]

Hi Bertolt,

by(test,INDICES=test$groupID,FUN=mean)

And today's a holiday in Switzerland, so stop working already ;-)

HTH
Stephan



Bertolt Meyer schrieb:
> 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
>