The exclusion of some colleges from NSC will induce measurement error in the dependent variable. If this error is not correlated with treatment (i.e., classical measurement error) then the true effect of class size on college enrollment will be larger than our observed effect by the proportion of enrollment that is missed (approximately 20 percent). This is because the true treatment effect is the sum of the observed treatment effect and the treatment effect of the unobserved college attenders(Bound, Brown, & Mathiowetz, 2001). However, if the measurement error in college attendance is correlated with assignment to treatment then our effect could be either  downward or upward biased. This would be the case, for example, if colleges attended  by marginal students are disproportionately undercounted by NSC.
注：
1、NSC:全国学生资料库
2、用有道直接翻译什么的就算了，我也会用的哦!