Within-person (or within-subject) effects represent the variability of a particular value for individuals in a sample. You see this commonly examined in repeated measures analysis (such as repeated measures ANOVA, repeated measures ANCOVA, repeated measures MANOVA or MANCOVA…etc). In these instances, a within person effect is a measure of how much an individual in your sample tends to change (or vary) over time. In other words, it is the mean of the change for the average individual case in your sample.
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