Many thanks - this is exactly what I was hoping for. Very helpful!

Best wishes,
Jeremy

On 15 Jun 2011, at 21:22, Ista Zahn-2 [via R] wrote:

I have to confess that plyr has made me lazy about remembering tapply,
by, aggregate et al., so I'm no help there. But if you want to use
plyr it's just

ddply(dat, .(V1), summarize, cor.v2.v3 = cor(V2, V3))

Best,
Ista

On Wed, Jun 15, 2011 at 10:31 AM, jfdawson <[hidden email]<x-msg://177/user/SendEmail.jtp?type=node&node=3600456&i=0>> wrote:

> I'm hoping there is a simple answer to this - it seems that there should be,
> but I can't figure it out.
>
> I have a matrix/data frame with three variables of interest - V1, V2, V3.
> One, V1, is a factor with x levels (x may be a large number); I want to
> calculate  the correlation between the other two (i.e. cor(V2,V3)) for each
> level, and store it as a vector of length x.
>
> I should think this should be possible using a function like tapply, but I
> cannot work out what the syntax would be - everything I have tried produces
> either an error, or just repeats the correlations between V2 & V3 on the
> whole matrix.
>
> Could anyone suggest what I should be doing?
>
> Many thanks,
> Jeremy
>
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--
Ista Zahn
Graduate student
University of Rochester
Department of Clinical and Social Psychology
http://yourpsyche.org<http://yourpsyche.org/>

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