School choice researchers and critics discuss the strengths and weaknesses of studies analyzing the effects of vouchers in "The Problem With Studying Vouchers," by D.W. Miller, Chronicle of Higher Education, July 13, 2001 (Article is available only to subscribers.)
(For more on this topic, see "Voucher Vortex," by David Glenn, Lingua Franca, May/June 2001 (not available at http://www.linguafranca.com, but you can order a copy of the magazine))