Abhi Sivasailam
I am writing this post from India, where I have been vacationing the past few weeks. One of the most striking features of the social organization here is the prevalence of private schooling. Selection bias aside, I have yet to meet a student who attends a public school. It is not uncommon for impoverished children and children of slum residents to spurn free public education in favor of private education. Recognizing this, I have been wondering how the increased share of private school students here affects levels of educational innovation, test scores, and other educational outcomes. Fortunately, I've just found an interesting new study that evaluates this effect. The authors, Martin West and Ludger Woessmann, purport to measure the treatment effect of the "share of schools that are privately operated" on PISA (Programme for International Student Assessment) scores. Of course, there is a problem here with determining causality. As they write:
Countries where more people choose to invest in private schools may have other attributes, such as higher income levels or a greater commitment to education, that lead to better achievement. If this is the case, any positive correlation between private schooling and student achievement could reflect a country’s income or educational commitment rather than any beneficial effects of competition. Or it may be the case that low-quality public schools increase the demand for private schooling. If so, then it could seem that competition lowered public school quality when in fact the causal connection could be in the opposite direction.

To solve for this, West and Woessmann exploit a technique that economists call an instrumental variables approach. Essentially, they found a new variable — the size of a country's Catholic population in 1900 — which is a useful predictor for the current share of schools that are private. Because the new variable is a good predictor of the variable they are really interested in (share of schools that are private), but is presumably not influenced by higher income levels or greater commitment to education, using this new variable is more useful for determining causality.

Their results (emphasis added):
Our results indicate that the share of schools that are privately operated has an economically and statistically significant positive effect on student achievement in mathematics, science, and reading, even after controlling for the current levels of Catholics and for the share of funding that privately operated schools receive from the government. Larger historical Catholic shares that translate into a ten percentage point larger private school sector today increase average student achievement on the math test by 9% of an international standard deviation. Science and reading achievement increase by roughly 5% of a standard deviation. These patterns are evident despite the fact that the contemporary share of Catholics in each country is negatively related to student achievement, suggesting that distinctive cultural features of traditionally Catholic countries are unlikely to be driving these results. Importantly, much of the positive effect of private school shares accrues to students in public schools, suggesting that the overall effect is not simply due to privately operated schools being more effective, but rather it reflects benefits of competition.

It's not clear how applicable these results are to private school competition within only the United States or within individual states such as Missouri. However, if the same pattern holds for educational institutions in Missouri, then policies that encourage private school competition, like vouchers or tuition tax credits, will have spillover benefits for public school students as well — not just the recipients of educational aid.

By the way, those readers who are now beginning to suspect that I spend most of my free time perusing the economics of education literature would be correct in that suspicion.

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Abhi Sivasailam