Analysis with and without litter taken into account. Nine pregnant female C57BL/6 mice were injected with 600 mg/kg VPA subcutaneously on embryonic day 13, and five control females received vehicle injections. Half of the animals in each condition were also injected with either a mGluR5 receptor antagonist (MPEP) or saline postnatally. Total locomotor activity in the open field over a 30 min period at 8–9 weeks of age is shown. There was a slight increase in activity due to MPEP, but it was not significant when differences between litters were ignored (A; two-way ANOVA, mean difference = 0.60, F(1,44) = 3.17, p = 0.082). Adjusting for litter removed unexplained variation in the data, allowing the small difference between groups to become statistically significant (B; mixed-effects model, mean difference = 0.64, F(1,32) = 7.19, p = 0.011). Note how the values in the second graph have less variability around the group means; this increased precision leads to greater power of the statistical tests. Lines go through the mean of each group and points are jittered in the x direction.