Why permutation tests are superior to t and F tests in biomedical research

A survey of 252 prospective, comparative studies reported in five, frequently cited biomedical journals revealed that experimental groups were constructed by randomization in 96% of cases and by random sampling in only 4%. The median group sizes ranged from 4 to 12. In the randomized studies in which measurements were made on a continuous scale, comparisons of location were made by t or F tests in 84% of cases, and by nonparametric, rank-order, tests in the remainder. Because randomization rather than random sampling is the norm in biomedical research and because group sizes are usually small, exact permutation or randomization tests for differences in location should be preferred to t or F tests.