Wilcoxon signed-rank test An inferential statistical data analysis used when comparing two related samples, matched samples, or repeated measurements on a single sample to assess whether their population mean ranks differ (i.e. it is a paired difference test). It can be used as an alternative to the paired Student's t-test, t-test for matched pairs, or the t-test for dependent samples when the population cannot be assumed to be normally distributed. http://en.wikipedia.org/wiki/Wilcoxon_signed-rank_test Person: Jie Zheng, Oliver He non-parametric test