On 26/10/14 16:40, David Doyle wrote:
Hello,
I'm trying to run kruskal test on some data but the p values seemed way too
low. So I tried it on some similar data and still got p-value =
1.611e-09. I'm sure it is a simple mistake but I can't figure it out.
Below is my data and code. Could it be because there are some miss data /
NAs in the data set?? If so, could some one point me towards a solution??
Thank you for your time.
David
mydata <-read.csv("http://doylesdartden.com/R/test.csv";, sep=",")
kruskal.test(mydata, AMMONIA~Well)
Kruskal-Wallis rank sum test
data: mydata
Kruskal-Wallis chi-squared = 36.3952, df = 1, p-value = 1.611e-09
Thank you for providing a clear question and an easily reproducible example.
The problem is your syntax for the call to the kruskal.test() function.
It should be:
kruskal.test(AMMONIA ~ Well, data=mydata)
This gives a p-value of 1 (which agrees with wilcox.test); t.test()
gives a p-value of 0.7958) so harmony is restored to the universe.
IMHO there is a bit of a design flaw in kruskal.test(); it should have
thrown an error, given your syntax. The wilcox.test() function *does*
throw an error.
cheers,
Rolf Turner
--
Rolf Turner
Technical Editor ANZJS
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