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Self-Selection Bias at Charter Schools

Whenever headlines proclaim that charter schools outperform traditional public schools, corporate reformers are jubilant.  That's because the tax code allows equity funds and banks that invest in charter schools in underserved areas to get an extremely generous tax credit. The results posted by the 32 Success Academies, the largest charter chain in New York City, will understandably make their day ("Eva Moscowitz just got more toxic," New York Daily News, Aug. 18).  

Ninety-four percent of Success students were rated proficient in math,  compared with only 35 percent of students in New York City's other public schools.  In English, 64 percent of Success were proficient, compared with 29 percent elsewhere in the city.  These are impressive results, and the teachers deserve high praise.

But headlines don't tell the entire story. Comparing charter schools with traditional public schools is virtually meaningless because of the role that self-selection bias plays. A fairer way is to compare the performance of students admitted to charter schools with those who applied but were turned away for one reason or another.  In other words, lottery winners are compared to lottery losers.  But as I pointed out in a letter to the editor in The New York Times, this strategy can be applied only to charter schools that are popular enough to require a lottery and that keep accurate records ("Inside the World of Charter Schools," Mar. 3, 2013).

Moreover, the turnover of teachers in New York City's charter schools (26-33 percent) is far greater than that at traditional public schools in New York City (13-16 percent).  The reasons are unclear, but I suspect that the demands made on teachers and the lack of a union explain the churn ("Teachers at Charter Schools Struggle, Too," In These Times, Aug. 20).

I support parental choice.  However, I can't emphasize enough that not all children have parents who are engaged enough - if at all - in their education to take advantage of the opportunities open to them.  As a result, they invariably become collateral damage.  With charter schools expected to enroll five million by the end of this decade, we have to decide as a society if we are willing to live with this situation.

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The opinions expressed in Walt Gardner's Reality Check are strictly those of the author(s) and do not reflect the opinions or endorsement of Editorial Projects in Education, or any of its publications.

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