Friday, May 22, 2009

Class size and student performance: major pitfalls in measurement

The literature on the impact of class size on student performance is largely inconclusive. Yet, there is still a public perception that size matters. Is the econometrician wrong? Miguel Urquiola and Eric Verhoogen argue that there is a subtle issue with class size that can seriously affect estimates.

Many studies exploit the following discontinuity. Jurisdictions mandate that class sizes cannot be larger than some number. For schools, this means when the number of students overtake a multiple of this number a new class needs to be opened. The consequence is a sudden decrease in the average number of students in classes, because the number of students is close to continuous, the number of classes is, however, a low integer. So far so good.

But the problem is with the endogenous reaction to this discontinuity. Schools are very reluctant to open a new class just because of one student, and will find ways around this, for example by increasing tuition. Also, parents select schools according to the number of students in classes. The big question is whether this matters in any significant way. Urquiola and Verhoogen build a model of schools maximizing profits and parents choosing schools and test it with data from Chile. There, private, subsidized schools are for profit and thus fit nicely with the model and indeed the above mentioned effects are important.

The schooling environment in Chile is particular, because a large portion of schools are for-profit. This may not be true elsewhere, but the fact is that schools are still reacting to incentives, and parents, too. So while the data and motives may not be as easy to model as in Chile, this needs to be taken into account for any estimate of the impact of class size.


Education economist said...

And that said, there are still many papers published on the class size published with simple linear regressions or even simple correlations. At least that should no longer be tolerated in Economics journals, but Education journals are still accepting them...

Independent Accountant said...

Many years ago a class size student performance study was done in New York City (NYC). The "educationalists" were shocked with the result: the kids in larger classes did better.
What the "educationalist" fools didn't know is that in those days, about 45 years ago, NYC had a target class size of 32. It assigned better performing students to larger classes, up to 40. Why? They had fewer behaviorial problems. Result: larger classes "led" to better student performance.
I have no use for these class size studies. For my money, all they are are attempts by the "teacher-educator-consultant-diversity" complex to get the public to spend more money on schooling the ineducable. I want to be clear on this: I don't think any of these studies is worth a nickel.