However, they also reflect other characteristics that are associated with socio-economic status but that are not measured by the PISA index. For example, at the system level, high socio-economic status is related to greater wealth and higher spending on education. At the school level, higher socio-economic status is associated with a range of characteristics of a community that might be related to student performance, such as a safe environment and the availability of quality educational resources, such as public libraries or museums.