On national and international assessments, students attending French-language schools in Ontario usually perform worse than students attending English-language schools. Interpreting these results is challenging because the French- and English-language schools differ both in prescribed curriculum and in how the curriculum is taught. In addition, the French- and English-language versions of the tests and scoring procedures sometimes differ. Even how students in the French- and English-language schools take the tests may differ. Finally, the populations of students differ in important ways. In this paper, we illustrate these challenges using results from the 2001 Progress in International Reading Literacy Study.