A November, 2013, report of the What Works Clearinghouse stated that it could find “no studies of Reading Mastery that fall within the scope of the Beginning Reading review protocol [and] meet What Works Clearinghouse (WWC) evidence standards” (WWC, 2013b, p. 1). This NIFDI technical report documents substantial errors in the WWC’s compilation of studies to examine and in the interpretations of individual studies. Effect sizes (Cohen’s d) were computed for results of more than three dozen studies identified by the WWC but rejected for analysis. All of these studies conformed to standard methodological criteria regarding valid research designs and would have been accepted in scholarly reviews of the literature. The average effect size associated with Reading Mastery (RM) was .57. This value is more than twice the .25 level traditionally used to denote educational significance. The results replicate meta-analyses that have found strong evidence of the efficacy of Reading Mastery. Given the high rate of error in this and other WWC reports consumers are advised to consult reviews of studies in the standard research literature rather than WWC summaries.