Two articles by the same authors estimate the effects of principals on student achievement. Both report school-level, value-added Texas state test results for school principals, controlling for gender, race, free and reduced-price lunch, and earlier academic performance. They find large to small effect sizes depending on the model examined. The study is severely limited by methodological issues, including the age of the data (12-18 years old) and failure to account for several variables: experience as a principal, the effectiveness of comparison principals, district assignment policies, nonlinear effects of principals on schools, other influences on high-poverty schools, and non-principal influenced teacher mobility. Regarding principal mobility, the authors failed to account for district policies and career mobility patterns. Finally, the authors could not estimate the effects of first-year principals. These flaws raise serious questions about the actual effect sizes of principals on student test scores and thus the validity of the analyses. The report is most useful for methodological discussions about value-added estimates for principals, the validity of different models, and principal effects on teacher and principal turnover. The most important policy-relevant conclusion that can be derived from this report is that estimating principal effectiveness is simply not possible given current methodology and sample size restrictions. Thus, such estimates should not be used to evaluate principals.