The study used an experimental design with random assignment of schools to treatment and control conditions within each participating district. Schools remained in the same treatment condition for both years of the study. The difference in outcomes between the treatment schools and the control schools can be interpreted as the effect of the study's PD model relative to "business as usual" in each participating district.
Midway through the first implementation year, results from the NCEE study of PD in early reading became available (see Garet et al. 2008). The results showed that although the single year of PD tested in the study had a statistically significant impact on some dimensions of teacher knowledge and instructional practice at the end of the year in which the PD was implemented, the PD did not produce a statistically significant impact on student achievement and did not produce a statistically significant impact on teachers' knowledge, teachers' instructional practices, or student achievement in the year following the year of the PD. That is, the study had no statistically significant impact on student achievement, and the impact of the PD on teachers' knowledge and instructional practice was not sustained.
Based on this information, NCEE elected to explore the effect of extending the implementation of the seventh-grade mathematics PD to two years. Because of resource constraints, the second year of PD was offered in only half of the originally participating districts.