Thank you! It's what I want :)

But I can't make it work. I'm working with R version 2.7.1 under Debian
GNU/Linux. Let me paste my results:

> pairwise.wilcox.test(SAND,Organ,p.adj="bonf") -> a

    Pairwise comparisons using Wilcoxon rank sum test

data:  SAND and Organ

        FlowerA FlowerB FlowerC FlowerD LeafA
FlowerB 0.00062 -       -       -       -
FlowerC 0.00062 0.00432 -       -       -
FlowerD 0.00062 0.00062 0.00062 -       -
LeafA   0.00614 0.00614 0.00614 0.00614 -
LeafC   0.00062 0.00062 0.00062 0.00062 0.32425

P value adjustment method: bonferroni
Warning messages:
1: In wilcox.test.default(xi, xj, ...) :
  cannot compute exact p-value with ties
2: In wilcox.test.default(xi, xj, ...) :
  cannot compute exact p-value with ties
3: In wilcox.test.default(xi, xj, ...) :
  cannot compute exact p-value with ties
4: In wilcox.test.default(xi, xj, ...) :
  cannot compute exact p-value with ties
5: In wilcox.test.default(xi, xj, ...) :
  cannot compute exact p-value with ties

> multcompLetters(a,Letters=c(letters,LETTERS,"."))
Error in do.call(compare, list(x, threshold)) :
(list) object cannot be coerced to 'double'

I though it was because the bad format of the argument "a" (I think the most
similar input may be "A square, symmetric matrix with row names"), so I
tried with one valid input produced by a Tukey test (valid as input but not
as an analysis of my data because its non-normality) with the aim of
checking if this was the problem.

> TukeyHSD(aov(SAND~Organ)) -> b
  Tukey multiple comparisons of means
    95% family-wise confidence level

Fit: aov(formula = SAND ~ Organ)

$Organ
                      diff          lwr         upr     p adj
FlowerB-FlowerA -10.392222 -12.12877102  -8.6556734 0.0000000
FlowerC-FlowerA -14.692222 -16.42877102 -12.9556734 0.0000000
FlowerD-FlowerA  -5.280000  -7.01654880  -3.5434512 0.0000000
LeafA-FlowerA    -3.478889  -5.21543769  -1.7423401 0.0000044
LeafC-FlowerA    -2.508889  -4.24543769  -0.7723401 0.0011579
FlowerC-FlowerB  -4.300000  -6.03654880  -2.5634512 0.0000000
FlowerD-FlowerB   5.112222   3.37567342   6.8487710 0.0000000
LeafA-FlowerB     6.913333   5.17678453   8.6498821 0.0000000
LeafC-FlowerB     7.883333   6.14678453   9.6198821 0.0000000
FlowerD-FlowerC   9.412222   7.67567342  11.1487710 0.0000000
LeafA-FlowerC    11.213333   9.47678453  12.9498821 0.0000000
LeafC-FlowerC    12.183333  10.44678453  13.9198821 0.0000000
LeafA-FlowerD     1.801111   0.06456231   3.5376599 0.0379548
LeafC-FlowerD     2.771111   1.03456231   4.5076599 0.0002720
LeafC-LeafA       0.970000  -0.76654880   2.7065488 0.5655196

> multcompLetters(b,Letters=c(letters,LETTERS,"."))
Error in do.call(compare, list(x, threshold)) :
(list) object cannot be coerced to 'double'
Además: Warning message:
In if (class(x) == "dist") x <- as.matrix(x) :
  la condición tiene longitud > 1 y sólo el primer elemento será usado (I'm
sorry for the spanish warning, it means that the condition has length>1 so
only the first element will be used)

Any idea of what was the mistake I did?

Thank you again!

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