The Wilcoxon rank sum test, also known as Mann-Whitney U-test, doesn’t make any assumption concerning the statistical distribution of words in a corpus (Wilcoxon 1945, Mann & Whitney 1947). It is based on a comparison of a sum of rank orders of texts in two text collections. The rank orders of texts are defined according to a frequency of a target word, without considering to which of both corpora this text belongs (see Lijffijt 2014). In our implementation, it sums up the frequencies per segment of document; for this reason, we consider it to be a dispersion-based rather than a frequency-based measure. 

References

Lijffijt, Jefrey, Terttu Nevalainen, Tanja Säily, Panagiotis Papapetrou, Kai Puolamäki, and Heikki Mannila, ‘Significance Testing of Word Frequencies in Corpora’, Digital Scholarship in the Humanities, 31.2 (2014), pp. 374–97, http://doi.org/10.1093/llc/fqu064
Paquot, Magali, and Yves Bestgen, ‘Distinctive Words in Academic Writing: A Comparison of Three Statistical Tests for Keyword Extraction’, in Corpora: Pragmatics and Discourse, ed. by Andreas H. Jucker, Daniel Schreier, and Marianne Hundt (Brill | Rodopi, 2009), doi:10.1163/9789042029101_014
Woolson, R. F., ‘Wilcoxon Signed-Rank Test’, in Wiley Encyclopedia of Clinical Trials, ed. by Ralph B. D’Agostino, Lisa Sullivan, and Joseph Massaro (John Wiley & Sons, Inc., 2008), p. eoct979, doi:10.1002/9780471462422.eoct979
Zimmerman, Donald W., and Bruno D. Zumbo, ‘Relative Power of the Wilcoxon Test, the Friedman Test, and Repeated-Measures ANOVA on Ranks’, The Journal of Experimental Education, 62.1 (1993), pp. 75–86, http://doi.org/10.1080/00220973.1993.9943832
Mann, H. B., and D. R. Whitney, ‘On a Test of Whether One of Two Random Variables Is Stochastically Larger than the Other’, The Annals of Mathematical Statistics, 18.1 (1947), pp. 50–60, http://doi.org/10.1214/aoms/1177730491
Wilcoxon, Frank, ‘Individual Comparisons by Ranking Methods’, Biometrics Bulletin, 1.6 (1945), p. 80, http://doi.org/10.2307/3001968