Quasi-experiments in second language research: Key concepts and recommendations

The current introductory chapter proposes a second language field-specific definition of quasi-experimental research designs. It begins by outlining key concepts related to quasi-experimental research, including the notion of causality, and emphasizes the importance of variable control for causal inference. The chapter addresses various types of validity—specifically, internal, external, and ecological validity—as well as the threats to each and the tensions that may arise between them in particular experimental contexts. It then examines counterfactual evidence in relation to control and comparison groups, emphasizing the necessity of isolating variables to achieve experimental control and strengthen inferences. The relative strengths of experimental and quasi-experimental designs are also considered. Drawing on these considerations, we propose a more descriptive, field-specific definition and implementation procedure of quasi-experimental research that reflects the nature of second language research.