Why selection bias is the most powerful force in education – the ANOVA

Or consider exclusive public high schools like New York’s Stuyvesant, a remarkably competitive institution where the city’s best and brightest students compete to enroll, thanks to the great educational benefits of attending. After all, the alumni of high schools such as Stuyvesant are a veritable Who’s Who of high achievers and success stories; those schools must be of unusually high quality. Except that attending those high schools simply doesn’t matter in terms of conventional educational outcomes. When you look at the edge cases – when you restrict your analysis to those students who are among the last let into such schools and those who are among the last left out – you find no statistically meaningful differences between them. Of course, when you have a mechanism in place to screen out all of the students with the biggest disadvantages, you end up with an impressive-looking set of alumni. The admissions procedures at these schools don’t determine which students get the benefit of a better education; the perception of a better education is itself an artifact of the admissions procedure. The screening mechanism is the educational mechanism.