A quasi-experimental design shares the logic of a true experiment, with a treatment group, comparison group, and pre/post measurement, but lacks random assignment. Participants are allocated to conditions based on pre-existing groupings, typically intact classes. This is the most common design in classroom SLA research because random assignment of individual students is rarely feasible in school settings.
Schools assign students to classes for administrative, logistical, or pedagogical reasons, not research purposes. Researchers must work with these intact groups. The result: groups may differ systematically before the treatment begins (e.g., one class may have higher motivation, a different teacher, or more hours of prior instruction). These pre-existing differences are the central threat to Internal Validity.
| Design | Structure | Notes |
|---|---|---|
| Non-equivalent control group | Intact treatment + intact comparison, pre/post-test | The workhorse of classroom SLA research |
| Time-series | Repeated measures before and after treatment | Useful when no comparison group is available |
| Counterbalanced | Groups receive treatments in different orders | Controls for sequence effects |
| Regression discontinuity | Assignment based on a cut-off score | Rarely used in SLA but strong for causal inference |
Because groups are not randomly formed, several confounds can explain post-treatment differences:
Researchers strengthen quasi-experimental studies through:
Norris & Ortega (2000) and Plonsky (2013) called for SLA researchers to report effect sizes, confidence intervals, and detailed participant information to make quasi-experimental findings interpretable and meta-analysable.