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Ed Knows Policy

EKP -- a local (Washington, DC) and national blog about education policy, politics, and research.

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Friday, July 14, 2006

Comparisons of public, private schools on NAEP test

I'm sure there will be a lot of interest in the latest comparison by NCES (researchers Henry Braun, Frank Jenkins, and Wendy Grigg) of public and private schools, including breakdowns by types of religious schools.

But the problem is that such comparisons are descriptive, and confound sorting effects with school quality. Also, it's NAEP, so you have test score levels in grades 4 and 8 only, not growth over a specified period using year-to-year performance changes. The test takers could be recently enrolled in public or private sector with scores that reflect where they've been for their whole school careers.

Continued on the flip...

But this is all I had to read:

Second, the data are obtained from an observational study rather than a randomized experiment, so the estimated effects should not be interpreted in terms of causal relationships. In particular, private schools are "schools of choice." Without further information, such as measures of prior achievement, there is no way to determine how patterns of self-selection may have affected the estimates presented. That is, the estimates of the average difference in school mean scores are confounded with average differences in the student populations, which are not fully captured by the selected student characteristics employed in this analysis.


Of course, "schools of choice" means that you are comparing kids as much as you are comparing schools. If you notice, the private school advantage shrinks when you start to control for student background characteristics, in some cases flipping the result to favor public schools. That should not be too suprising.

1 Comments:

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10:01 PM  

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