This study was conducted to determine whether scheduling interventions make a difference in student achievement. It examined the effects of scheduling types on Iowa Tests of Educational Development (ITED) scores in Iowa high schools. Student performance, as measured by ITED scores, was used to compare 4 x 4 block-scheduled schools, A/B alternating-day block-scheduled schools, 8-period day scheduled schools, and the Iowa state norms. An analysis of covariance was used as the analytical procedure. The ANCOVA factored in both school size and gender. The battery of ITED scores used for comparison included reading, mathematics, science, social studies, and composite scores. This study suggests that there is no significant difference in student performance as measured by the Iowa Tests of Educational Development in Iowa schools using a 4 x 4 or an A/B block-schedule. No significant difference was found in all but three of the tests when comparing students in schools using a traditional 8-period day schedule and students in either a 4 x 4 or an A/B block scheduled school.;One unique aspect of this study was the comparison of the ITED scores in the year prior to implementation of a block schedule with the 1999 ITED scores of the same school. Only schools that had been on block scheduling two or more years were selected for the study. This longevity component suggests that the ITED mean scores of schools will increase, although not significantly, after the conversion to an A/B block schedule. The same results were not produced with a 4 x 4 schedule.;Another pattern that developed throughout the course of the hypothesis testing was the fact that the variability of scores in the block schools was consistently greater than in the traditionally scheduled schools. When factoring in gender, the range in standard deviations primarily was due to the wide differences in males' mean scores.;Size of the school had little effect on student performance on the ITED. There was a positive correlation between mean scores on the ITED and school size. As schools increased in size, the ITED mean scores rose, but except in one hypothesis test, these differences were not significant.