Visualising litter-to-litter variation. The residuals represent the unexplained variation in the data after the effects of VPA and MPEP have been taken into account; they should be pure noise and therefore not associated with any other variable. However, the standard analysis (A) shows that when residuals are plotted against litter (x-axis) there are large differences between litters. In other words, there is another factor affecting the outcome besides the experimental factors of interest. The variance of the residuals (grey points on the right) is high ( = 1.29). The proper analysis (B) reduces the unexplained variation in the data by 61% ( = 0.50; p < 0.001), which can be seen by the narrower spread of the grey points around zero, and the large differences between the litters have been removed. This reduction in noise allows smaller true signals to be detected. Error bars are SEM. Litters F and L only have one observation and thus no error bars.
Lazic and Essioux BMC Neuroscience 2013 14:37 doi:10.1186/1471-2202-14-37