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

[Dimitris Rizopoulos]

one option is aggregate(), e.g.,

test <- as.data.frame(cbind(c(rep(1,5),rep(2,5)), rnorm(10), rnorm(10)))
names(test)[1] <- "groupID"
aggregate(test[c("V2", "V3")], list(test$groupID), mean)


I hope it helps.

Best,
Dimitris

-- 
Dimitris Rizopoulos
Biostatistical Centre
School of Public Health
Catholic University of Leuven

Address: Kapucijnenvoer 35, Leuven, Belgium
Tel: +32/(0)16/336899
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      http://perswww.kuleuven.be/dimitris_rizopoulos/


Quoting Bertolt Meyer <bmeyer at 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 at sozpsy.uzh.ch
> tel:   +41446357282
> fax:   +41446357279
> mob:   +41788966111
>
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