[R] pairwise.t.test

[peter dalgaard]

On 10 Feb 2015, at 11:45 , Thomas Chesney <Thomas.Chesney at nottingham.ac.uk> wrote:

> I'm using pairwise.t.test() with 6 groups, but the dataset the 6 groups are in actually contains 24 groups.
> 
> When I run the test with all 24 groups the results I'm getting between Group 1 and Group 2 are very different from the results I'm getting between Group 1 and Group 2 when I split off the 6 groups and run the test. When only 6 groups are included the results are highly significant but with all 24 groups the results are insignificant.
> 
> I'm not adjusting p.values. The code I'm running is:
> 
> res <- pairwise.t.test(dat$Adopt, dat$Group, p.adj="none")
> 
> I thought the above code would run exactly the same test on Groups 1 and 2, regardless of how many groups are in the dataset. Can anyone explain what's going on?

Check out pool.sd on the help page.

-pd


> 
> Thank you,
> 
> Thomas Chesney
> 
> 
> 
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-- 
Peter Dalgaard, Professor,
Center for Statistics, Copenhagen Business School
Solbjerg Plads 3, 2000 Frederiksberg, Denmark
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Email: pd.mes at cbs.dk  Priv: PDalgd at gmail.com