While voucher advocates claim their research is comparable with research showing the benefits of class-size reduction, the data show otherwise. Voucher data are inconsistent and inconclusive and, despite recent favorable publicity, do not yet make the case for voucher programs. By contrast, a wealth of data using very large samples provide strong evidence that class-size reduction clearly raises student achievement.

http://epsl.asu.edu/epru/point_of_view_essays/cerai-00-28.htm