Gary Coffman wrote:

Wisconsin, Ohio, Pennsylvania, Texas, Florida, and
the District of Columbia all have, or are starting, pilot
voucher programs. Wisconsin's is the oldest, and
has had positive measurable results in terms of
improved test scores for Milwaukee children in the
program.

And where was the equivalent control group, with similar selection
criteria, taken from in the public system?

You have to be careful about looking at data about comparing schools.
Saying the voucher system works by saying some select few that would
have done well anyway, did better than the average of those left behind
isn't much of a case.

Fitch