This study examined the percentage of task-sampling variability in performance assessment via a meta-analysis. In total, 50 studies containing 130 independent data sets were analyzed. Overall results indicate that the percentage of variance for (a) differential difficulty of task was roughly 12% and (b) examinee's differential performance of the same task was approximately 26%. Based on moderator analysis, research design and subject area were significant predictors of variance component for tasks ([sigma][superscript 2][subscript t]) and variance component for person-task interaction ([sigma][superscript 2][subscript pt]). Mathematics and listening were associated with relatively high proportion of task-sampling variability whereas foreign language was least affected by this variability. To reduce task variation, some techniques are recommended: (a) incorporating more facets when possible, (b) using a crossed rather than a nested design, and (c) incorporating occasion as a facet. (Contains 3 tables.)