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I am trying to find the critical value for the Wilcoxon one-sample signed-rank test. Currently, I can find the value using tables. I looked at qwilcox() in R, but it appears that this gives the critical values for the Wilcoxon two-sample test (or Mann–Whitney test). Is there a function in R which I can use to compute this critical value?

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You can use the qsignrank() function. Example:

> qsignrank(.025, 10, lower.tail=FALSE)
46

This means that for a sample size of 10 and a two-sided test with a significance level of 5%, the test statistic must be greater than 46 (i.e., 47 or greater) to be statistically significant. Example data:

> set.seed(1)
> x = rnorm(10, .5)
> wilcox.test(x)

    Wilcoxon signed rank test

data:  x
V = 47, p-value = 0.04883
alternative hypothesis: true location is not equal to 0

Here the test statistic is 47, and significant at the 5% level.

Note that for a two-sided test, the test statistic returned by qsignrank() is the larger of the two possible test statistics. For example, wilcox.test(-x) gives a test statistic of 8, which can be transformed into 47 by $\frac{10\cdot 11}{2}-8$.

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