Power calculations for VPA experiments. Panel A shows how power changes as the number of animals per litter increases from one to eight (x-axis) and the number of litters per group increases from three to ten (different lines). It is clear that increasing the number of animals per litter has only a modest effect on power with little improvement after two animals. A two-group study with three litters per group and eight animals per litter (2 × 3 × 8 = 48 animals) will have only a 30% chance of detecting the effect, whereas a study with ten litters per group and one animal per litter (2 × 10 × 1 = 20 animals) will have almost 80% power and also use far fewer animals. Panel B shows the same data, but presented differently. Power for different combinations of litters and animals per litter is indicated by colour (red = low power, white = high) and reference lines for 70%, 80%, and 90% power are indicated. Note that these specific power values are only relevant for the locomotor activity task with a fixed effect size and will have to be recalculated for other outcomes. However, the general result (increasing litters is better than increasing the number of animals per litter) will apply for all outcomes.