On determining sample size in experiments involving laboratory animals.

Scientists using laboratory animals are under increasing pressure to justify their sample sizes using a "power analysis". In this paper I review the three methods currently used to determine sample size: "tradition" or "common sense", the "resource equation" and the "power analysis". I explain how, using the "KISS" approach, scientists can make a provisional choice of sample size using any method, and then easily estimate the effect size likely to be detectable according to a power analysis.