In case anyone missed the release of this study this week, Rob Warren of the University of Minnesota has a new study comparing high school graduation rates in Milwaukee’s voucher program and public schools. The bottom line is that students graduate at much higher rates in the voucher program.
Warren is careful to emphasize that he cannot draw causal inferences from this work. That is, the voucher students graduate at higher rates than public students, but he can’t say whether the voucher program caused their higher graduation rate. That kind of conclusion can only be drawn from a study that compares apples to apples. With Pat Wolf I am involved in an evaluation that will be able to produce a graduation rate comparison of matched samples of voucher and public students, but results are still a few years down the road.
Jay P. Greene's Blog 
[…] very smart Jay Greene also points out the need for caution: Warren acknowledges “he can’t say whether the voucher program caused their higher […]