The No Child Left Behind Act requires states to publish high school graduation rates for public schools; the U.S. Department of Education is currently considering a mandate to standardize high school graduation rate reporting. However, no consensus exists among researchers or policymakers about how to measure high school graduation rates. We use longitudinal data tracking a cohort of students at 82 Texas public high schools to assess the precision of three widely used high school graduation rate measures: Texas's official graduation rates and two competing estimates based on publicly available enrollment data from the Common Core of Data. Our analyses show that these widely used approaches yield highly imprecise estimates of high school graduation and persistence rates. We propose several guidelines for using existing graduation and persistence rate data and argue that a national effort to track students as they progress through high school is essential to reconcile conflicting estimates. (Contains 9 notes, 5 tables, and 1 figure.)