1. Threats to Conclusion Validity

2. Low statistical power Low statistical power Violated assumptions of statistical tests Violated assumptions of statistical tests Fishing and the Error Rate problem Fishing and the Error Rate problem Low reliability of measures Low reliability of measures Poor reliability of treatment implementation Poor reliability of treatment implementation

3. Threats to Conclusion Validity Random irrelevancies in the setting Random irrelevancies in the setting Random heterogeneity of respondents Random heterogeneity of respondents

4. Improving Conclusion Validity: General Guidelines Want good statistical power (>.8) Want good statistical power (>.8) Want good reliability Want good reliability Want good implementation Want good implementation

5. Improving Conclusion Validity: Specific Suggestions Pretest and posttest on same scales Match before or after random assignment (but not in place of random assignment) Use covariates Increase reliability, especially of dependent measures Adjust raw scores for unreliability Estimate desired effect magnitude in advance Each person as own control Present effect and standard error Select homogeneous samples