[R] wilcox.test and ties
Timo.Makelainen at helsinki.fi
Fri Jul 19 19:03:19 CEST 2002
On 19 Jul 2002, at 18:59, Timo Mäkeläinen wrote:
> On 19 Jul 2002, at 15:16, Swanepoel, Christiaan Edward wrote:
> > Hello,
> > when performing the wilcox.test on a sample (n=100) with the
> > following frequency distribution: Value | Frequency
> > ---------------------------------------- 0 | 10 0.1 | 88 1 | 1
> > 2.6 | 1
> > I get the following error message:
> > Error in uniroot(wdiff, c(mumin, mumax), tol = 1e-04, zq =
> > qnorm(alpha/2, :
> > f() values at end points not of opposite sign
> > The error only occurs when I specify that the confidence interval
> > should be computed.
> > Could anybody suggest how I can compute the confidence interval for
> > this sample? Is the problem connected with the large number of ties
> > and if so, how can I handle these ties?
> >From the help:
> "By default (if exact is not specified), an exact p-value is computed
> if the samples contain less than 50 finite values and there are no
> ties. Otherwise, a normal approximation is used."
> The error message tells that a normal approximation is being
Suggesting that some ties have been correctly recognised.
On the other hand, I would rather not rely on a normal
> approximation here. With so few observations differing from 0.1, a
> hand computation might be easy for a high confidence level.
> In general, for the correct handling of ties entering data like yours
> as round( x, 1) can be tried.
Better, look at the code to see how they are handled.
> > Thanks in advance.
> > Regards
> > Chris
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> Timo Makelainen, Department of Mathematics
> University of Helsinki, P.O. Box 4
> FIN-00014 Helsinki
> Tel.: +358-9-19122863
> Fax: +358-9-19123213
> E-mail: Timo.Makelainen at Helsinki.Fi
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Timo Makelainen, Department of Mathematics
University of Helsinki, P.O. Box 4
E-mail: Timo.Makelainen at Helsinki.Fi
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