[R] wilcox.exact() problem
Achim Zeileis
Achim.Zeileis at wu-wien.ac.at
Tue Oct 27 16:31:09 CET 2009
On Tue, 27 Oct 2009, David Croll wrote:
> Dear R friends,
>
> here I write again about the wilcox.exact() problem. I want to compare two sets of categorical data, and in one case it says "negative length vectors not allowed", and in the other one I get the error "cannot allocate vector of length ...".
>
> On http://rapidshare.com/files/298621893/wilcox.exact_trouble.Rdata.html you can download the data that cause the trouble. (Attention: 10 Downloads only, so you may have to send me an e-mail so I can mail you the Rdata file directly.)
>
>> library(exactRankTests)
> Package ?exactRankTests? is no longer under development.
> Please consider using package ?coin? instead.
>> wilcox.exact(group_1,group_2,exact=T)
> Error in .Call("cpermdist2", ma = as.integer(m), mb = as.integer(col), :
> negative length vectors are not allowed
>> wilcox.exact(group_3,group_4,exact=T)
> Error in .Call("cpermdist2", ma = as.integer(m), mb = as.integer(col), :
> cannot allocate vector of length 1621367542
>
> 1621367542 is the same as 2^30.59, so the memory can't be allocated on my 32-bit computer.
>
> Can somebody point out the problem to me?
The sample size is too large (~4000 observations per group) to solve this
problem exactly. The error message could maybe be improved, but the
message is clear: This is too large to deal with.
However, this is not a problem. With several thousand observations,
standard normal approximations should work sufficiently well. And if you
don't believe it, then you can look at approximate solutions that draw a
sufficiently large number of permutations. Both is easily available when
using wilcox_test() in "coin" as the startup message of "exactRankTests"
suggests.
hth,
Z
>
> Kind regards,
>
>
> David
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