[R] What are ties? Wilcox u-test

Uwe Ligges ligges at statistik.tu-dortmund.de
Wed Apr 30 09:47:56 CEST 2008

Dirkheld wrote:
> Hi,
> When I execute a Wilcox u-test on two variables I receive a warning :
> 'cannot compute exact p-value with ties'
> - What are ties? What does this mean for my data?

If you have two identical values in your data, these are called ties.
Now the ranks are not unique anymore and hence exact p-values cannot be 
And since you do not know this, it might be worth to note that the 
Wilcoxon test also assumes a non-skewed distribution.

> - Is that a problem for significance testing?

If there are just a few ties, you should not worry, but re-check how 
your data looks like. It is a warning, not an error message.

> - is there a way to overcome this problem?

If you need to, use an appropriate test for your data.

> I have different threads in this forum but it hard to find what the exact
> meaning of this warning message is.

Read some textbook?

Uwe Ligges

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