Stable individual differences refer to the consistent, underlying physiological or psychological traits—such as variations in individuals' nervous systems, muscle reflex speeds, or cognitive abilities—that cause some participants to naturally perform faster or slower than others. In an analysis of variance, these inherent variations contribute to the variability within groups. While they inflate the mean squares within groups ($MS_W$) in between-subjects designs, they can be statistically measured and partitioned out of the $MS_W$ in within-subjects designs, resulting in a more sensitive statistical test.