[R] Bootstrap or Wilcoxons' test?

[Murray Cooper]

Charlotta,

I'm not sure what you mean when you say simple linear
regression. From your description you have two groups
of people, for which you recorded contaminant concentration.
Thus, I would think you would do something like a t-test to
compare the mean concentration level. Where does the
regression part come in? What are you regressing?

As for the Wilcoxnin test, it is often thought of as a
nonparametric t-test equivalent. This is only true if the
observations were drawn, from a population with the
same probability distribution. The null hypothesis of
the Wilcoxin test is actually "the observations were
drawn, from the same probability distribution".
Thus if your two samples had say different variances,
there means could be the same, but since the variances
are different, the Wilcoxin could give you a significant result.

Don't know if this all makes sense, but if you have more
questions, please e-mail your data and a more detailed
description of what analysis you used and I'd be happy
to try and help out.

Murray M Cooper, Ph.D.
Richland Statistics
9800 N 24th St
Richland, MI, USA 49083
Mail: richstat at earthlink.net

----- Original Message ----- 
From: "Charlotta Rylander" <zcr at nilu.no>
To: <r-help at r-project.org>
Sent: Friday, February 13, 2009 3:24 AM
Subject: [R] Bootstrap or Wilcoxons' test?


> Hi!
>
>
>
> I'm comparing the differences in contaminant concentration between 2
> different groups of people ( N=36, N=37). When using a simple linear
> regression model I found no differences between groups, but when 
> evaluating
> the diagnostic plots of the residuals I found my independent variable to
> have deviations from normality (even after log transformation). Therefore 
> I
> have used bootstrap on the regression parameters ( R= 1000 & R=10000) and
> this confirms my results , i.e., no differences between groups ( and the
> distribution is log-normal). However, when using wilcoxons' rank sum test 
> on
> the same data set I find differences between groups.
>
>
>
> Should I trust the results from bootstrapping or from wilcoxons' test?
>
>
>
> Thanks!
>
>
>
> Regards
>
>
>
> Lotta Rylander
>
>
> [[alternative HTML version deleted]]
>
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