In trying to gauge the success of Milwaukee's school choice experiment in boosting student achievement, researchers in the past have compared the learning of students in the program with that of their peers who remained in the public schools. Taking a different tack, Harvard economist Caroline Hoxby looks at the achievement levels of public school students alone.

She is taking seriously a main rationale for the voucher program: that choice creates a competition with the public schools and, as a consequence, prods them to reform. Hoxby's finding is that there seems to be fact to back the theory. The more competition a public elementary school got from the voucher program, she says, the more it tended to improve.

School choice remains a contentious issue, and Hoxby's study will certainly not be the final word. But her work does reinforce an impression shared by many observers, including this newspaper's Editorial Board, that school choice has prompted innovations within the Milwaukee Public Schools.

The voucher program rests chiefly on two philosophical underpinnings:

As a matter of equity, low-income parents deserve a range of school options, similar to what other parents enjoy by virtue of their relative prosperity. Competition would stir giant, moribund, monopolistic, rules-encrusted, reform-resistant school bureaucracies into better serving students.

Hoxby's work shores up the latter rationale. Indeed, it can be argued that raising achievement for students left in MPS is even more important than lifting it for choice students. After all, the MPS students outnumber the latter, 10 to one. In that respect, the competition theory amounts to a powerful argument for choice.

By the way, in an interview with the Journal Sentinel, Hoxby attributed MPS improvement to more than immutable laws of economics. She noted that the school system chose to respond to the competition by bettering itself - for which MPS deserves credit.