This study addressed the question: Why do parochial school students in Rhode Island tend to perform better than public school students in relation to national norms on the Iowa Tests of Basic Skills (ITBS)? Data for examination of school setting and population factors were collected from interviews with teachers and administrators, a short questionnaire was administered to teachers, and computer tape records of demographic information were collected during the Rhode Island statewide testing program. Data on psychometric and test-intrinsic factors were obtained from the ITBS technical manuals, phone conversations with ITBS author A. N. Hieronymus, and scores from the statewide testing program. A variety of factors were found to contribute to the performance differences. Parochial schools had higher pupil/teacher positive in parochial schools. The curriculum of the parochial schools stressed basic procedures, parental support, and student motivation were more positive in parochial schools. The curriculum of the parochial shcools stressed basic skills while the public school curriculum was more varied. A higher percentage of females attended parochial schools, and parochial students' fathers had higher occupational status. It was also found that differences in ITBS score distributions occurred at the middle and lower end of the distribution, were greatest in the language subtest, and least in mathematics. (Author/BW)