I read with interest the recent Patch article about charter schools and teacher evaluations. I'd like to share the perspective I gained from ten years on the San Carlos School Board as regards charter and traditional public school performance.
Virtually every analysis I've seen on this topic ignores selection bias. An implicit assumption is made that the children attending public charter schools and public traditional schools reflect the same population.
At first blush that sounds reasonable. Both are open to all, neither charges tuition, and both are funded by public money. So their populations must be the same, right?
But that ignores several well known differentiating factors. Most charter schools make parent/family volunteering a necessity. In fact, until a recent change in charter law, many actually required "volunteering."
While all public schools encourage volunteerism, the emphasis on it at charters is much greater. This makes charters less attractive to families for whom volunteering would be difficult or impossible.
In addition, at least some charter schools discourage students with challenging learning issues from applying. By law, charter schools cannot turn away an applicant on that basis. But in practice things aren't so simple.
I personally witnessed a famous charter proponent counsel a charter oversight board on how to dance around the legal prohibition so as to save money. Another pretty much wrote "we're not for everyone" right into their charter.
Charters generally have to be sought out by families. They are not the "default option" when it comes to enrolling your child in public school. This, too, introduces a selection bias.
As another famous charter school proponent told me, "Only people who are willing to sacrifice for their kids have the time, energy and inclination to seek us out and go through our application process."
Does all of this matter in terms of differentiating the student populations of charter and traditional public schools? I believe the evidence shows it does. The authors of Freakonomics and SuperFreakonomics -- enjoyable books on "things we think we know" -- reported on the best study of selection bias in public education I've ever come across.
Several years ago the Chicago public school system shifted to an open enrollment process for all its high schools. Any middle school student could apply to any high school, regardless of service area boundaries. Schools that were oversubscribed held lotteries to determine who got in. No one got in just because of where they lived. The authors compared the educational outcomes of the winners and losers of those lotteries.
There was no significant difference in performance between the kids who won the lottery and the ones who lost. Apparently, being the kind of student who was motivated to make a choice regarding schooling meant you were the kind of student who would do well in school. It wasn’t the school that was important, it was wanting to do well in school.
This is consistent with analyses of standardized test results in California. Between 70 percent and 75 percent of the differences in standardized test scores between elementary schools can be explained by just three factors: the proportion of students from college-educated families; the proportion of students speaking English natively; and the proportion of students from poor economic circumstances. Every other factor – differences in funding, the impact of teacher performance, etc. -- is in the remaining 25 – 30 percent.
Does that mean we should forget about teacher performance? Of course not. But our debate would be more meaningful – more likely to create real positive change – if we remember to account for selection bias when making comparisons, and focus on the elephant in the room: where kids come from, and what they bring to school.
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