One challenge in understanding "what works" in education is that effect sizes may not be comparable across studies, raising questions for practitioners and policymakers using research to select interventions. One factor that consistently relates to the magnitude of effect sizes is the type of outcome measure. This paper uses study data from the What Works Clearinghouse to determine average effect sizes by outcome measure type. Outcome measures were categorized by whether the group who developed the measure potentially had a stake in the intervention (non-independent) or not (independent). Using meta-analysis and controlling for study quality and intervention characteristics, we find larger average effect sizes for non-independent measures than for independent measures. Results suggest that larger effect sizes for non-independent measures are not due to differences in implementation fidelity, study quality, or intervention or sample characteristics. Instead, non-independent and independent measures appear to represent partially but minimally overlapping latent constructs. Findings call into question whether policymakers and practitioners should use research based on non-independent measures when they are ultimately responsible for improving outcomes on independent measures. [This paper was published in "Journal of Research on Educational Effectiveness."]