Dear Julia,
Try also

x=read.table(textConnection("ID    value
111     5
111     6
111     2
178     7
178     3
138     3
138     8
138     7
138     6"),header=TRUE)
 closeAllConnections()
attach(x)

do.call(rbind,tapply(value,ID, function(x){
res=c(mean(x,na.rm=TRUE),var(x,na.rm=TRUE))
names(res)=c('Mean','Variance')
res
}
)
)

HTH,

Jorge




On Thu, Sep 11, 2008 at 1:45 PM, liujb <liujulia7@yahoo.com> wrote:

>
> Hello,
>
> I have a data set that looks like this.
> ID    value
> 111     5
> 111     6
> 111     2
> 178     7
> 178     3
> 138     3
> 138     8
> 138     7
> 138     6
> .
> .
> .
>
> I'd like to calculate the mean and var for each object identified by the
> ID.
> I can in theory just loop through the whole thing..., but is there a easier
> way/command which let me calculate the mean/var by ID?
>
> Thanks,
> Julia
> --
> View this message in context:
> http://www.nabble.com/Calculate-mean-var-by-ID-tp19440461p19440461.html
> Sent from the R help mailing list archive at Nabble.com.
>
> ______________________________________________
> R-help@r-project.org mailing list
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> PLEASE do read the posting guide
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> and provide commented, minimal, self-contained, reproducible code.
>

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