The purpose of this study was to examine the relationship of two implementation variables (source of technical support and time of implementation) to first grade reading achievement across three naturally occurring conditions over a six year time span. All first graders (n=40,000) who were in the Baltimore City Public School System from 1997-98 through 2002-03 were included in the study. Two of the conditions involved implementation of Direct Instruction (DI) curriculum programs, one with technical support from a purveyor associated with the curriculum’s developer (the National Institute for Direct Instruction) and the other with a group not associated with the developer. Eleven schools were in the first group, and five schools were in the second. The third condition involved implementation of Open Court (103 schools). The study examined changes in average first grade achievement, measured by the Comprehensive Test of Basic Skills (vocabulary, comprehension, and composite scores) over this six year time-span. Combining data across all years (1998-2003) students in NIFDI-supported schools had significantly higher achievement than students in the Control schools on all three measures of achievement and significantly higher comprehension scores than students in the Other DI schools. When the socio-economic characteristics of the schools were controlled, the students in the NIFDI-supported schools had significantly higher achievement scores than students in both the Other DI schools and the control schools on all measures of achievement. First grade students in all 3 groups of schools had higher achievement scores in 2003 than in 1998, but the increase was significantly larger for students in NIFDI-supported schools than for students in the other schools. On average, first grade composite reading achievement scores in the NIFDI-supported schools increased by 113 percent from 1998 to 2003, while achievement in the control schools and the Other DI schools increased by 56 percent or less. The magnitude of the effect on first grade achievement from attending a NIFDI-supported school was statistically significant and substantively large. At the end of the study period (2003) the effect size (Cohen’s d) of attending a NIFDI-supported school versus attending an Other DI school on composite achievement was .63. The effect of attending a NIFDI-supported school versus a Control school was .82. Thus, the use of Direct Instruction curricula, the purveyor of technical support and the time of implementation all had significant effects on reading achievement. (Copyright © 2011, National Institute for Direct Instruction (NIFDI). All rights reserved).