School vouchers, particularly since the landmark U.S. Supreme Court ruling on the Cleveland program, are one of the most contentious issues in American education. Seemingly contradictory results across available studies have caused confusion among diverse audiences. The authors suggest that these divergent findings are, in part, due to three factors, unique to voucher research, associated with choice. How these factors combine for various comparison groups are examined and how the combinations likely impact estimations of voucher effect are discussed. The authors close by suggesting that the most complete understanding of the impact of vouchers on students requires the use of multiple comparison groups. (Contains 1 table.)