Differentiated instruction is commonly believed to be critical to improving the quality and efficiency of teachers' instructional repertoires (Fischer & Rose, 2001; Tomlinson, 2004). Tomlinson (2000) describes differentiation in four domains: content, process, product, and learning environment. Content differentiation involves varying instructional topics, for example, that students within a classroom would receive. Process differentiation involves teaching different students at different levels of difficulty. Product differentiation involves assigning different tasks to different students. Learning environment differentiation includes using different instructional groupings for students or clustering students based on ability. The purpose of this paper is to present a measure of instructional differentiation derived from teacher checklist data and apply it in the context of a randomized experiment that is part of a phased rollout of Diagnostic Assessment Tools statewide in Indiana.