Broward County Public Schools (BCPS) has identified low student achievement in statistics in the middle grades as a critical concern. It has also expressed concern that statistics instruction does not meet the level of cognitive complexity described in the state curriculum standards. This randomized controlled trial in 40 BCPS middle schools analyzed the extent to which the Supporting Teacher Enactment of the Probability and Statistics Standards (STEPSS) program--a 20-day curriculum unit designed to support teaching and learning of the state curriculum standards in probability and statistics at grade 7, along with four days of professional development for teachers--affects student understanding of statistics and statistics instruction compared with practice as usual. Key findings include: (1) Students in STEPSS program schools scored higher on the Levels of Conceptual Understanding in Statistics test than students in practice-as-usual schools; (2) Statistics instruction in STEPSS program schools received higher scores on the Instructional Quality Assessment than statistics instruction in practice-as-usual schools; and (3) Statistics instruction in STEPSS schools consistently involved the program's lessons provided in the statistics replacement unit. [For the full report, see ED610166. For the appendixes, see ED610168.]