[R] How to compute within-group mean and sd?

[R. Michael Weylandt]

This is a very commonly used paradigm in R and there are many
different answers, one would be tapply(). You could also look at the
plyr package.

Michael

On Sat, Mar 24, 2012 at 11:32 AM, reeyarn <reeyarn at gmail.com> wrote:
> Hi, I want to run something like
>  SELECT firm_id, count(*), mean(value), sd(value)
>  FROM table
>  GROUP BY firm_id;
>
> But I have to write a for loop like
>  for ( id in unique(table$firm_id ) {
>    print(paste(  id, mean(table[firm_id == id, "value"])  ))
>  }
>
> Is there any way to do it easier? Thanks :)
>
>
> Best,
> Reeyarn Lee
>
>        [[alternative HTML version deleted]]
>
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