The study sample consisted of 5,255 offenders who were serving custodial sentences in prisons across England and Wales who completed 1 of 2 accredited cognitive skills program during fiscal year 2001/2002. The relationship between a battery of assessment measures, tutor experience, drop-out rates, audit observations, and the quality of program delivery was explored. The test battery of assessment measures designed to determine short-term impact on targeted behaviors and attitudes focused on locus of control, impulsivity, empathy, socialization, social problem solving, and the eight cognitive thinking patterns believed to maintain criminal behaviors and lifestyle. The study found a short-term impact on the majority of assessment measures across both programs. Patterns of change were broadly similar across gender, age, and ethnic groups. The impact of the program was significantly greater for high-need inmates and inmates at sites where tutoring was delivered more often. Tutor delivery rates were also found to be related to drop-out rates and the quality of program delivery. At the program site level, positive correlations were observed between the quality of delivery and other measures of the site's performance. Thus, the accreditation system has successfully identified key aspects of program delivery that apparently influence the short-term effectiveness of the programs. 4 tables and 30 references