This means that education systems with greater fairness in education outcomes, as measured by the percentage of the variation in student performance explained by socioeconomic status, lend to show smaller performance differences between students from different socio-economic groups, as measured by the average change in performance scores associated with a one-unit change on the PISA index of economic, social and cultural status. That is, most countries show either steep, strong socio-economic gradients or flat, weak gradients. In these cases, universal policies tend to be most effective. These types of policies include changing curricula or instructional systems and/or improving the quality of the teaching staff.