[R] Multiple Comparisons-Kruskal-Wallis-Test: kruskal{agricolae} and kruskalmc{pgirmess} don't yield the same results although they should do (?)
greatest.possible.newbie
daniel.hoop at gmx.net
Fri Aug 3 11:33:15 CEST 2012
Thank you for your answer.
The p.adj argument in the kruskal()-function doesn't seem to change
anything... Not even the "bonferroni"-method although it is described as the
most conservative one (multiplying all p-values with the number of
comparisons). I suppose the kruskal()-function is not working properly...
On the other hand I doubt the method behind the kruskalmc()-function as this
function doesn't even turn out to detect significant differences between the
grouping variable (which is obviously a severe error).
Do you think it is justifiable to use the kruskal()-function without
p-adjustment, i.e. doing only pairwise tests like you can do with the
kruskal.test()-function although I obviously want to do multiple
comparisons?
kruskal(x[,1],x[,2],p.adj="bonferroni")
#Yields exactely the same results.
#Groups, Treatments and mean of the ranks
#a 11 304.4
#ab 9 296
#ab 7 286.6
#ab 8 278.2
#ab 10 268.7
#ab 2 250.6
#ab 6 242.9
#ab 1 242.1
#ab 3 239.4
#ab 5 228.8
#b 4 219.5
kruskalmc(x[,2],x[,2])
#Multiple comparison test after Kruskal-Wallis
#p.value: 0.05
#Comparisons
# obs.dif critical.dif difference
#[......]
#6-9 54.0 162.02688 FALSE
#6-10 69.5 159.04584 FALSE
#6-11 94.5 133.02196 FALSE
#7-8 18.0 160.00778 FALSE
#7-9 35.0 169.78370 FALSE
#7-10 50.5 166.94123 FALSE
#7-11 75.5 142.36796 FALSE
#8-9 17.0 165.54197 FALSE
#8-10 32.5 162.62538 FALSE
#8-11 57.5 137.28174 FALSE
#9-10 15.5 172.25281 FALSE
#9-11 40.5 148.56074 FALSE
#10-11 25.0 145.30369 FALSE
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