This article provides practical guidance for researchers who are designing studies that randomize groups to measure the impacts of educational interventions. The article (a) provides new empirical information about the values of parameters that influence the precision of impact estimates (intraclass correlations and R[superscript 2] values) and includes outcomes other than standardized test scores and data with a three-level structure rather than a two-level structure, and (b) discusses the error (both generalizability and estimation error) that exists in estimates of key design parameters and the implications this error has for design decisions. Data for the paper come primarily from two studies: the Chicago Literacy Initiative: Making Better Early Readers Study (CLIMBERS) and the School Breakfast Pilot Project (SBPP). The analysis sample from CLIMBERS comprised 430 four-year-old children from 47 preschool classrooms in 23 Chicago public schools. The analysis sample from the SBPP study comprised 1,151 third graders from 233 classrooms in 111 schools from 6 school districts. Student achievement data from the Reading First Impact Study is also used to supplement the discussion. (Contains 9 tables, 1 figure, and 16 footnotes.)