Siegel–Tukey test
a nonparametric test that evaluates any difference in variance across two independent samples to determine whether they represent two different populations. Data values for the two groups are first ordered in a single list from lowest to highest and then ranked in terms of alternate extremes, so that, for example, the highest and lowest values are both ranked 1, the next highest and next lowest 2, and so on. Finally, the ranked values in each group are summed. If final sums per group do not differ, there is no evidence that the dispersion differs across groups. The test has relatively low statistical power when data have a normal distribution.