R help - Jul 2008 - t-test for multiple variables

Dear list,

i try to do my first statistics with R.

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.

Thanks, Henning    
-- 

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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