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Measuring the dependent variable multiple times for each participant in a within-subjects design allows for a more refined calculation of within-groups variance ($$MS_W$$). Because stable individual differences can be quantified and subtracted from $$MS_W$$, the resulting denominator in the $$F$$-ratio is smaller. This lower $$MS_W$$ leads to a higher $$F$$-value, making the repeated-measures ANOVA a more sensitive statistical test.

Within-Groups Variance in Repeated-Measures ANOVA

If a study uses reaction time as a dependent variable, stable individual differences (such as variations in participants' nervous systems and muscles) will naturally cause some individuals to be faster or slower. In a between-subjects ANOVA, these differences add to the within-groups variance ($$MS_W$$), decreasing the $$F$$-ratio. In a repeated-measures ANOVA, these differences are measured and subtracted from $$MS_W$$, increasing the test's sensitivity.

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