Selection bias is a problem for mathematics education researchers interested in using observational rather than experimental data to make causal inferences about the effects of different instructional methods in mathematics on student outcomes. Propensity score methods represent 1 approach to dealing with such selection bias. This article describes general principles underlying propensity score methods and illustrates their application to mathematics education research using 2 examples investigating the impact of problem-solving emphasis in mathematics classrooms on students' subsequent mathematics achievement and course taking. Limitations of the method are discussed. (Contains 6 tables and 2 footnotes.)