[R] wilcox.test and ties

[Timo Mäkeläinen]

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 
attempted. 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.

> Thanks in advance.
> 
> Regards
> Chris
> 
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Timo


Timo Makelainen, Department of Mathematics
University of Helsinki, P.O. Box 4 
FIN-00014 Helsinki 
Tel.: +358-9-19122863 
Fax:  +358-9-19123213
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