This paper highlights the essential need for appropriate statistical design and randomisation in laboratory animal studies. Using an example of a 21 day weight gain study in mice, we show that without the use of an appropriate statistical design and randomisation, incorrect conclusions
may have been drawn. We used an experimental design that allowed comparisons to be made between five treatments that were free from systematic error. Two alternative designs that are practically attractive, yet had no statistical basis, are also described in this paper and the potentially
incorrect conclusions highlighted. The use of appropriate statistical design is ethical because it results in clear, unambiguous conclusions. Conclusions that may be biased or ambiguous will require verification by further research and this, in the long term, is contrary to the reduction element
of the Three Rs.