# Mann-Whitney U test for sample sizes 65 and 10 in Python

I would like to run the Mann-Whitney U for two groups with sizes 65 and 10, where I essentially want to compare scores on some measure between two different groups.

I have two questions regarding the mannwhitneyu function in python's scipy stats library:

• First of all, the documentation says:

Use only when the number of observation in each sample is > 20 and you have 2 independent samples of ranks. Mann-Whitney U is significant if the u-obtained is LESS THAN or equal to the critical value of U.

So I assume I can't use it since the size of my second sample is < 20. The only alternative I know is where both sample sizes < 20, where we manually calculate $U$, and then compare the test statistic against a table of critical values. However, the only tables I can find are where both sample sizes are < 20.

So is there a version of Mann-Whitney U I can use for these sample sizes (preferably with implementation in python)? Or should I use an entirely different test, such as Mood's Median test?

My second question is a bit more general regarding the mannwhitneyu in scipy:

• It seems to output the test statistic $U$ and a $p-$value. So if you had sample sizes greater than 20, and you wanted to determine whether this result is statistically significant, is the $p-value$ that is outputted, the one for normalised $U$? I.e. it is the $p-$value for $z = \frac{U-m_u}{\sigma_u}$ ? So can you just read off this $p-$value to determine the statistical significance of the difference between the two groups? If so, what is the point of the function outputting $U$? What would you do with that information?

• Thanks for your answer. Just to clarify, I'm just wondering if you know whether the $p$ value that is given in the output from scipy is the $p$ value associated with the normal distribution already? In other words, if I have observed that the mean of one group is greater than the other, can I just read off the $p$ value to determine it's statistical significance? – Terrence J Dec 15 '16 at 22:14
• I'm sorry, I'm not quite sure what you're asking. The actual distribution of the Mann-Whitney test statistic is discrete, not normal. The thing labelled p-value in the output will be an approximation to the correct p-value for the Mann-Whitney, using a normal approximation to the exact distribution. I was pointing out that as long as you don't go deep into the tail, the p-value obtained by using the normal approximation isn't so bad for sample sizes of 10 and 65. If mannwhitneyu won't do it, it's easy to compute the Z score directly and just call a normal-cdf function to obtain a p-value. – Glen_b -Reinstate Monica Dec 15 '16 at 22:29