This is a quick update to my post of April 25.  Just yesterday the U. S. News released its newest list of the best high schools in the United States.  The list includes a separate ranking for charter schools, which are ranked on the basis of several indicators.

The indicator with the highest weight (30%), is “College Readiness,” which is the proportion of 12^thgraders who took and passed at least one AP or IB exam.  This indicator applies only to those students who enrolled in 9^th grade or later AND were retained through 12^thgrade.  These are the “survivors.”  All students who left the school prior to the administration of these exams in 12^thgrade–and there were many–are excluded from the calculation.  You won’t find a purer example of survivorship bias.

Charter schools are also ranked on “Math and Reading Proficiency” and “Math and Reading Performance,” which are both weighted at 20%.  Proficiency is the aggregated scores on state math and reading assessments that students may be required to pass for graduation.  This does not apply to California, because state graduation exams are not required.  It is not known how the 20% weight that would have been applied to this criterion is redistributed among the others.

Math and Reading Performance is how scores on state assessments compare to U.S. News’ expectations, given the student demographics of the school.  (They apparently use some kind of “value added” calculation, but I don’t have the space to get into that here.) In California, these assessments are administered late in 11^thgrade, so would measure only those students who did not transfer out before the time of administration.  More survivorship bias.

The graduation rate is weighted at 10% and is defined as the proportion of entering 9^thgraders who graduated four years later in 2018.  This is a curious way of putting it, because it implies that U. S. News tracks ALL 9^thgraders who enroll in a charter school, including those who leave the school prior to 12^thgrade.  They don’t.  That would require them to track outcomes for every student who transfers out of a charter school.  Actually, their numbers reflect the graduation rate for only the survivors.

The four-year cohort graduation rate for all California public schools in 2018 was 83%, and all of the graduation rates of the charter schools ranked reported by U. S. News exceed that rate.  At the same time, the statewide retention rate for the same cohort was 99%, and NONE of the ranked charter schools exceeded that rate.  The school with the highest graduation rate, at 100%, is Stockton Unified Early College Academy, which also has the third lowest retention rate:  77%.  Over all, the retention rates ranged from 58% (Stockton Collegiate International Academy) to 98% (Leadership Public Schools in Hayward), but only four exceeded 90% while six were below 80%. The charter school that was ranked #1 in California and #12 nationally—Preuss School USD—had a retention rate of only 82% and a graduation rate of 95%.