Practice effects, improvements in test performance from baseline to retest, vary by task. Some tasks are more resistant to learning with repeated use, while others show modest to significant improvement when a retest is conducted at a smaller interval. The Mobile Toolbox validation studies have examined 14-day test-retest intervals, and efforts are being made to publish the results of 7-day and 21-day test-retest intervals. Researchers conducting repeated assessments should plan to model and account for potential learning effects in their analysis and study designs.