Retention and graduation rates tend to track admission selectivity (and, to a lesser extent, choices of major and other characteristics of students besides family income). One might still be able to compare retention and graduation rates compared to expectations adjusted for admission selectivity and other characteristics to see if a given college has a treatment effect on them, but the differences are likely smaller than comparisons of raw retention and graduation rates may imply. For students from lower income families, such treatment effect may be in part due to affordability (cost and financial aid).