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. Should they want to be able to detect a smaller effect they can increase their provisional sample size and recalculate the effect size. This is simple, does not need any software and provides justification for the sample size in the terms used in a power analysis.
Keywords: 3Rs; ethics and welfare; policy; reduction; sample size; statistics; techniques.