# Nonparametric Tests

Several nonparametric tests are available. As usual, this section mentions only a few possibilities. Currently, these refer to an outcome variable that indicates ranks (or that can, and should, be ranked, such as a non-normal metric variable), and a grouping variable. Note that if your data do not represent ranks, Stata will do the ranking for you.

## Two groups

For two groups, you might use Wilcoxon's rank sum test (which is equivalent to Mann and Whitney's U test), or the median test. The commands for these tests go like this:

ranksum income, by(sex)

median income, by(sex)

For the median test, option `exact`

is available that computes an exact test according to Fisher.

## More than two groups

In this case, you might use the Kruskal-Wallis test. If the groups represent some kind or order, a test for trend across groups is available.

kwallis income, by(firmsize)

nptrend income, by(firmsize)

© W. Ludwig-Mayerhofer, Stata Guide | Last update: 21 Dec 2010