Advice: Mann-Whitney or Normal Distribution Test


Interval (non-normal) vs. Dichotomous


A. Small to medium sized samples:

MANN-WHITNEY U TEST: (also known as Wilcoxon's rank sum test). This is listed under the nonparametric methods section of the analysis menu. The test investigates the tendency for observations in one group to exceed the observations in the other group.


B. Large samples (n > 100) that are not clearly "non-normal":

Z (NORMAL DISTRIBUTION) TEST FOR THE COMPARISON OF MEANS. This is listed in the parametric methods section of the analysis menu. With large samples that are not clearly "non-normal" we can assume that the means are normally distributed and that the variances are good estimates of their population values. The definition of a large sample is somewhat debateable but > 100 is a reasonably safe level. You will find some authors quoting > 50 and even > 30 as large samples but we favour a more conservative approach.