R help - Jan 2012 - kruskal wallis post hoc?

Dear all, 

I run a kruskal wallis test and found significant results. Then, I conducted all
pairwise comparisons and found no significant results. Could anyone please give
me a hint as to why this happens or redirect me towards a specific web page
where I can find more info? (I used alpha=5% and made no bonferroni or other
correction for the pairwise comparisons)
Thank you

 
Dr. Iasonas Lamprianou
Department of Social and Political Sciences
University of Cyprus
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Try to use "kruskalmc" in the package pgirmess

I give one of my results

> kruskal.test(rojos~mes)
Kruskal-Wallis chi-squared = 132.3091, df = 3, p-value < 2.2e-16
> kruskalmc(rojos~mes)
Multiple comparison test after Kruskal-Wallis 
p.value: 0.05 
Comparisons
              obs.dif critical.dif difference
abril-julio 134.52815     56.84018       TRUE
abril-junio  53.42185     58.73584      FALSE
abril-mayo   58.96383     59.73167      FALSE
julio-junio 187.95000     44.40539       TRUE
julio-mayo   75.56432     45.71446       TRUE
junio-mayo  112.38568     48.05106       TRUE



-----
Mario Garrido Escudero
PhD student
Dpto. de Biolog?a Animal, Ecolog?a, Parasitolog?a, Edafolog?a y Qca. Agr?cola
Universidad de Salamanca
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Hi Iasonas ,
This is a stat question and not an R question.
But the general answer is that it could happen :)

The R question would have been if their is a Tukey HSD for kruskel.test,
the answer is yes:
http://stats.stackexchange.com/questions/17342/is-there-a-nonparametric-equivalent-of-tukey-hsd
But if you didn't get any significant result from the pairwise comparison,
I would say that the post hoc correction wouldn't help you (it could be
that the reason for this significance is based on some weird contrast...)

Best,
Tal





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On Thu, Jan 12, 2012 at 8:35 AM, Iasonas Lamprianou
<lamprianou@yahoo.com>wrote:
> Dear all,
>
> I run a kruskal wallis test and found significant results. Then, I
> conducted all pairwise comparisons and found no significant results. Could
> anyone please give me a hint as to why this happens or redirect me towards
> a specific web page where I can find more info? (I used alpha=5% and made
> no bonferroni or other correction for the pairwise comparisons)
> Thank you
>
>
> Dr. Iasonas Lamprianou
> Department of Social and Political Sciences
> University of Cyprus
>        [[alternative HTML version deleted]]
>
>
> ______________________________________________
> R-help@r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>
>
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