If children from disadvantaged backgrounds cannot attend high-performing schools because of financial constraints, then school systems that offer parents more choice of schools for their children will necessarily be less effective in improving the performance of all students. For example, cross-country analysis shows that in the systems where more students repeat a grade, the impact of students' socio-economic status on their performance is stronger. Furthermore, trends analysis shows that stratification is negatively related to systems' overall performance. Students in schools where no ability grouping is practiced scored eight points higher in mathematics in 2012 compared to their counterparts in 2003, while students in schools where ability grouping is practiced in some or all classes had lower scores in PISA 2012 than their counterparts in PISA 2003. In contrast, in comprehensive systems, schools must find ways of working with students from across the performance spectrum. These different incentive systems may help explain the greater level of equity achieved in systems that use stratification less.