The purpose of this quasi-experimental, ex-post facto study was to determine through quantitative analysis of longitudinal data, the effects of homogeneous differentiation on reading achievement. Specifically, the study sought to determine the achievement differences in reading of students who were taught using a within-class grouping method or a between-class grouping method. The study population consisted on students from a Mississippi school district who began first grade from the 2006-2007 school year through the 2009-2010 school year. Students who had been assessed over a three-year on the DIBELS, STAR, and the MCT2 assessment were eligible for inclusion in the study. A refined population sample of 240 subjects was identified. The data was analyzed using a multivariate analysis of variance and a repeated measures analysis. The results of the MANOVA indicated a significant statistical difference in achievement based on instructional grouping format. The results of the repeated measures analysis revealed no significant statistical differences in grouping format over time. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]