[R] t test problem?

Vito Ricci vito_ricci at yahoo.com
Wed Sep 22 11:44:03 CEST 2004

Hi Liu,

I'd suggest you to use non-parametric tests (see 
such as:
wilcox.test() in stats package
pairwise.wilcox.test() in stats package

and see the result tou got (significancy/non
significancy) and compare it with t test result;

Parametric Test   Analogous Non-Parametric test 

Student T-test   Wilcoxon Rank Sum Test 
Paired t-test    Wilcoxon Signed Rank Test or the Sign

to test normality you can use:

shapiro.test() in stats package 

if you decide to use log scale, you must use this for
both samples.


You wrote:

Hi, Many thanks for your helpful comments and
suggestions. The attached are the data in both log10
scale and original scale. It would be very grateful if
you could suggest which version of test should be
By the way, how to check whether the variation is
additive (natural scale) or multiplicative (log scale)
in R? How to check whether the distribution of the
data is normal? 
PS, Can I confirm that do your suggestions mean that
in order to check whether there is a difference
between x and y in terms of mean I need check the
distribution of x and that of y in both natual and log
scales and to see which present normal distribution?
and then perform a t test using the data scale which
presents normal distribution? If both scales present
normal distribution, then the t tests with both scales
should give the similar results?
Thanks again.

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