School systems that continue to differentiate among students in these ways need to create appropriate incentives to ensure that some students are not "discarded" by the system. This does not necessarily mean that if stratification policies were changed, students in stratified systems would have better instrumental motivation to learn, since PISA does not measure cause and effect. However, policy makers in highly stratified systems need to consider not only the equity aspect of education outcomes but also non-cognitive outcomes, such as students' attitudes towards learning. The scope of evaluations and assessments is not only limited to student assessments, most schools use various forms of evaluations, such as self-evaluations, external school evaluation and teacher appraisals as well.