Third party observers have been found to significantly impair neuropsychological test performance on measures of attention, verbal memory, verbal fluency, and cognitive symptom validity. One measure of the importance of a research-based finding for clinical practice is effect size. Effect sizes were calculated for selected social facilitation literature and empirical studies of the impact of a third party observer on formal neuropsychological measures. The average effect size estimate found for the social facilitation research was large. Effect size estimates associated with findings from the third party observer research were, on average, medium for memory measures and small for motor and attention/executive measures. These findings indicate that the presence of an observer during a neuropsychological evaluation should be expected to have a clinically meaningful impact on an examinee's test performance, with memory measures particularly vulnerable.