Biostatistics with R An Introductory Guide for Field Biologists

After a general introduction to nonparametric tests, we review two useful nonparametric tests for comparing two samples. The Mann-Whitney test is a counterpart of the two-sample t test, but it uses the ranks of recorded values instead. Despite this test often being described as a test of the differences between mean values, this only applies when both samples come from distributions where both distribution curves are of the same shape. The Wilcoxon test for paired observations corresponds to the parametric paired t test. We also introduce permutation tests, which represent another group of non-parametric methods for hypothesis testing. The methods described in this chapter are accompanied by a carefully-explained guide to the R code needed for their use.