Experimental and quasi-experimental designs are used in educational research to establish causality and develop effective practices. These research designs rely on a counterfactual model that, in simple form, calls for a comparison between a treatment group and a control group. Developers of educational practices often assume that the population from which control groups are drawn is unchanging in its behavior or performance. This is not always the case. Populations and study samples can change over time--sometimes dramatically so. We illustrate this important point by presenting data from 5 randomized control trials of the efficacy of Kindergarten Peer-Assisted Learning Strategies, a supplemental, peer-mediated reading program. The studies were conducted across 9 years and involved 2,591 students. Findings demonstrate a dramatic increase in the performance of control students over time, and suggest the need for a more nuanced understanding of the counterfactual model and its role in establishing evidence-based practices.