In this article, the authors illustrate that populations and study samples can change over time. They present data from 5 randomized control trials of the efficacy of Kindergarten Peer-Assisted Learning Strategies, a supplemental, peer-mediated reading program. Findings demonstrate a dramatic increase in the performance of control students over time. The results suggest the need for a more nuanced understanding of the counterfactual model and its role in establishing evidence-based practices.