Thank for help!

My ''problem'' is a little bit complicated. I have a dataset of trees (five 
tree species) and I need to calculate if there are the significant differences 
in the period of suppressed growth among tree species (length in years, e.g. 1, 
2, 3, 6, 10, 50, 80, etc.). Because data are not normally distributed (Levene 
test), my idea was to use kruskalmc test. 
 
So, my question is, how to evaluate the differences in the duration of 
suppressed growth among groups? Is ''bootstrap confidence interval'' for each 
tree species right solution? 

thanks, OV




________________________________
From: Thomas Lumley <tlum...@uw.edu>
To: Omphalodes Verna <omphalodes.ve...@yahoo.com> 
Cc: R Help <r-help@r-project.org> 
Sent: Sunday, December 16, 2012 11:59 PM
Subject: Re: [R] kruskalmc, significant differences while median values are the 
same


On Sat, Dec 15, 2012 at 10:04 PM, Omphalodes Verna <omphalodes.ve...@yahoo.com> 
wrote:

Dear list!
>
>I work with multiple Kruskal-Wallis test (kruskalmc, package pgirmess), which 
>evaluates differences in medians among groups (5 groups). A result of a test 
>is significant differences among some groups, while median values are the same 
>for 4 groups (using tapply). Why?
>
>

The Kruskal-Wallis test *doesn't* evaluate differences in medians, so there is 
quite likely nothing wrong in a formal sense.

However, this does suggest that your groups may not be stochastically ordered, 
which means the results of the Kruskal-Wallis test could be quite misleading.  
I'd suggest that you at least look at pairwise Wilcoxon tests to make sure the 
direction agrees with what the Kruskal-Wallis test implies. Box plots might 
also be a good idea. 

Or, if you really want differences in medians, look at differences in medians. 
A permutation test or a bootstrap confidence interval is probably the best way 
to do this.

   -thomas

-- 
Thomas Lumley
Professor of Biostatistics
University of Auckland     

______________________________________________
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.

Reply via email to