The percentage of variance in student achievement that is explained by student SES--"poverty's power rating," as some call it--tends to be less among smaller schools than among larger schools. Smaller schools, we are told, are able to somehow disrupt the association between SES and student achievement. Using eighth-grade data for 215 public schools in Maine, I explored the hypothesis that this finding is in part a statistical artifact of the lower reliability of school-aggregated student achievement in smaller schools. This hypothesis was supported for mathematics achievement but seemingly not for reading achievement. Implications are discussed. (Contains 3 tables and 4 figures.)