to what extent is the association between school SES and student achievement
consistent for all students regardless of their individual SES?
to what degree does student achievement increase in a linear fashion as school
SES increases (that is, is the relationship essentially linear, or does it depart
from linearity, perhaps suggesting that the relationship tapers off as school SES
increases or conversely, that there are thresholds that must first be crossed before
the strongly positive relationship between SES and academic performance is seen)?