One should not under-estimate the impact of the DC school voucher program on student achievement. According to the official announcement and the executive summary of the report, school vouchers lifted high school graduation rates but it could not be conclusively determined that it had a positive impact on student achievement.
Something about those findings sounds like a bell striking thirteen. Not only is the clock wrong, but the mechanism seems out of whack. How can more students graduate from private schools if they weren't learning more? Are expectations so low in the private sector that any one can graduate?
Peering beneath the press release and the executive summary into the bowels of the study itself one can get some, if not all the answers, to these questions.
Let's begin with the most important--and perfectly uncontested--result: If one uses a voucher to go to school, the impact on the percentage of students with a high school diploma increases by 21 percentage points (Table 3-5), an effect size of no less than 0.46 standard deviations. Seventy percent of those who were not offered a school voucher made it through high school. That is close to the national average in high school graduation rates among those entering 9th grade four years earlier. As compared to that 70 percent rate among those who wanted a voucher but didn't get one, 91 percent of those who used vouchers to go to private school eventually received a high school diploma.
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