[R] Calculating Kendall's tau
S Ellison
S.Ellison at LGCGroup.com
Thu Apr 2 16:06:01 CEST 2015
> I am analyzing trend using Mann-kendall test for 31 independent sample, each
> sample have 34 years dataset. I supposed to find Kendall “tau” for each
> sample. The data is arranged in column wise (I attached the data).To find
> Kendall tau, I wrote R script as:
> ...
> Anyone can tell me how can I get orderly displayed “tau” value?
Usually, in R, a hypothesis test returns an object, and you can extract an individual element of that object.
MannKendall seems to be no exception. Looking at the help page, a MannKendall test returns...
" A list with class Kendall.
tau Kendall’s tau statistic
sl two-sided p-value
S Kendall Score
D denominator, tau=S/D
varS variance of S"
To get just tau, say something like MannKendalltau[i]<-MannKendall(y[,i])$tau
But your code is a bit of a mess....
MannKendalltau<- numeric(nc) simply makes MannKendalltau a single integer equal to nc; that doesn't look sensible when the next thing you do is treat MannKendalltau as a vector. R's been kind to you and extended MannKendalltau when you tried to add things to later, non-existent, elements, but it clearly wasn't the right thing to do. Look up ?numeric, and then look up ?vector for next time you want to set up an empty vector.
Second, since MannKendall(y[,i]) ) returns a list object of class Kendall, MannKendalltau[i]<-MannKendall(y[,i]) assigns a whole object containing 5 values to each new element of your MannKendalltau. So your result is a list of lists.
Finally, you don’t need a loop at all. On a data frame, sapply would work nicely, so (although I've not tested it) something like
sapply(desta[,2:nc], 2, function(x) ManKendall(x)$tau)
ought to do the whole thing in one shot and package it nicely into a named vector while it's about it.
S Ellison
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