Hi Simon,

i tried your code, but it results in an error:

"error in var(x) : `x` is empty"

Do you have an idea what the reason might be?

Thanks a lot,

Henning

>             
> -------- Original-Nachricht --------
> Datum: Fri, 18 Jul 2008 11:37:24 +0100
> Von: Simon Anders <anders@ebi.ac.uk>
> An: Henning Wildhagen <HWildhagen@gmx.de>
> CC: r-help@r-project.org
> Betreff: Re: [R] t-test for multiple variables
> 
>             Hi Henning,
> 
> Henning Wildhagen wrote:
> > Given a dataframe of three columns, where col1 is of type factor, col 2 
> and 
> > col3 are numeric and pairs of observations i would like to perform a 
> paired 
> > t-test for each level of col1.
> > I would like to avoid specifying the levels of col1 manually because it 
> > seems to me as being stupid, however, i did not figure out how to tell 
> it R 
> > in an function. I tried with tapply, but it did not work.
> 
> That's what 'tapply' is meant for. Try this:
> 
> tapply( 1:nrow(df), df$col1, function(r)
>     t.test( df$col2[r], df$col3[r], paired=TRUE  ) )
> 
> Here, tapply calls the fucntion in the second line for each level of 
> col1, always passing the row indices of the rows with this level as 
> parameter 'r'.
> 
> HTH
>    Simon
> 
        
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