Charter Schools, Cohort Loss and Average Test Scores

Collegiate has a high degree of what the folks in the business (and charter schools are a business) call cohort loss. Cohort loss are all the kids who were admitted to a charter school but "didn't work out."

Sometimes the students don't like the school. Sometimes the students can't abide by the disciplinary code of conduct, which in some cases is absurdly picayune. Sometimes the school works on convincing the student and their parents that the schools is not a good fit and that little johnny has got to go. Sometimes the charter school keeps a 'got to go' list so teachers and administrators can more closely scrutinize the student's conduct and find a reason to give them the boot.

My view is that a school, funded with public funds, should take responsibility for every student they admit but I don't write the laws the govern the grand experiment we call charter schools.

Below is a chart of cohort loss, also known as pruning, at Collegiate Charter. On the same plot is the average test score of the remaining cohorts in the class.

If we're interested in comparing Collegiate average test scores with a public school, we must first adjust for cohort loss but how can we do that?

What if Boston Latin pruned 75% of its incoming class from middle school to 12th grade? Since they don't do this, can we do it on paper by averaging the top 25% test scores? You get the picture.