Eskola, S., Lauhikari, M., Voipio, H.-M. et al. 1999. Environmental enrichment may alter the number of rats needed to achieve statistical significance. Scandinavian Journal of Laboratory Animal Science 26(3), 134-144.

The aim of this study was to evaluate whether two enrichment items, an aspen tube and block, have any impact on commonly analysed clinical chemistry and physiological parameters of Wistar rats and - more importantly - on the extent of variation. Based on the variation detected, this study went on to calculate the smallest number of animals needed to detect an arbitrarily chosen treatment effect with and without these enrichment items. The enrichment items within litters increased the variation and hence the number of animals needed.