[R] significant test

[Romain Francois]

Le 31.10.2005 13:45, Prof Brian Ripley a écrit :

> On Mon, 31 Oct 2005, Romain Francois wrote:
>
>> Le 31.10.2005 11:22, Robert a écrit :
>>
>>> I have two groups of data and want to test how the
>>> mean of one group is significant different from the
>>> mean of the other group of data.
>>> which R function can be used?
>>> Thanks.
>>
>> t.test if you assume normality
>> wilcox.test otherwise
>
> Sorry, no.  

*I* should be sorry.
Thanks for the correction.

> The Wilcoxon test does NOT test a difference in means: its null 
> hypothesis is that the two samples came from the same continuous 
> distribution, a much narrower assumption.  (It is sensitive to 
> differences in variances, for example, and is probably closer to 
> testing a difference in medians than means where the shapes of the two 
> samples differ)
>
> The Welch two-sample t-test does test exactly the null hypothesis 
> stated under normality, and it is pretty insensitive to quite large 
> departures from normality.  It is unlikely (but possible) that means 
> are of primary interest in very-non-normal distributions.



-- 
visit the R Graph Gallery : http://addictedtor.free.fr/graphiques
+---------------------------------------------------------------+
| Romain FRANCOIS - http://francoisromain.free.fr               |
| Doctorant INRIA Futurs / EDF                                  |
+---------------------------------------------------------------+