The normal approximation to the Wilcoxon signed rank sum test statistic is used whenever the sample size is large (more than 30).

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The normal approximation to the Wilcoxon signed rank sum test statistic can be used when the sample size is large (more than 30). This is because the distribution of the test statistic approaches a normal distribution as the sample size increases, according to the central limit theorem. However, it is important to note that for smaller sample sizes, the normal approximation may not be accurate and other methods such as the exact test or Monte Carlo simulation may be necessary.

The Normal Approximation to the Wilcoxon Signed Rank Sum Test