At the local site, many students were not achieving necessary learning gains on state assessments. Administrators and teachers were concerned as to why students on the local level were falling short. Current trends emphasized differentiation of instruction in classrooms to meet individual student needs; however, classrooms generally adhere to a more whole group structure. Using flexible grouping to differentiate instruction allows students to be reinforced or challenged at their own skill level. The purpose of this qualitative study was to better understand if and how research-based data analysis practices are being used by third and fourth grade teachers to form flexible groups to differentiate instruction. This study was guided by Marzano's research connecting the importance of instructional methods with student achievement. The research questions examined teacher use of flexible grouping, formative assessment to drive instruction, and Marzano's identified best instructional strategies in lesson planning. Data were collected through a lesson plan checklist and semistructured interviews with teacher/practitioners, based on Marzano's framework. Participants included eight teachers (four third grade and four fourth grade teachers). Research findings suggested that teachers are aware of what differentiated instruction is but often struggle to find adequate time to group their students flexibly using the formative data that are collected in classrooms. Research reflected teachers' difficulty in finding adequate time for planning and preparation. This study may contribute to positive social change by providing district teachers and leaders with professional learning opportunities while expanding their repertoire of strategies. Educators may benefit from expanding their professional knowledge concerning flexible groups as it relates to student's skill levels. [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.]