Based on the case context, diagnose why the researcher cannot confidently conclude that the new teaching strategy caused the higher math scores. Explain how the lack of random assignment allowed specific preexisting factors to act as confounding variables, and clarify how these factors serve as alternative explanations for the outcome.
Case context: A researcher conducts a study to evaluate a new fraction-teaching strategy. She uses two preexisting third-grade classrooms: Ms. Williams's class (treatment group) and Mr. Jones's class (control group). Parents of highly motivated students had requested their children be assigned to Ms. Williams's class, while the principal assigned students with disciplinary issues to Mr. Jones's class because he is a stronger disciplinarian. At the end of the study, the researcher compares final math scores and finds that Ms. Williams's class performed significantly better.
Question: Based on the case context, diagnose why the researcher cannot confidently conclude that the new teaching strategy caused the higher math scores. Explain how the lack of random assignment allowed specific preexisting factors to act as confounding variables, and clarify how these factors serve as alternative explanations for the outcome.
Sample answer: The researcher cannot conclude that the teaching strategy caused the improvement because the groups were not randomly assigned, making them nonequivalent. Preexisting differences, such as higher student motivation in Ms. Williams's class and more disciplinary issues in Mr. Jones's class, act as confounding variables. These factors, along with differing teacher styles and classroom environments, serve as alternative explanations; the higher scores could be due to these differences rather than the teaching strategy.
Key points:
- Diagnosis of internal validity threats due to the lack of random assignment.
- Identification of preexisting student differences (motivation and disciplinary status) as confounds.
- Explanation of how teacher/classroom differences confound the teaching strategy's effect.
- Justification of how these confounding variables provide alternative explanations for the differences in final math scores.
Rubric: To receive full credit, the response must: (1) Correctly diagnose that causal conclusions are invalid due to lack of random assignment and nonequivalent groups. (2) Identify specific preexisting factors from the context (student motivation, student behavior/disciplinary issues) that differ between the classrooms. (3) Explain how these confounding variables serve as plausible alternative explanations for the final exam scores.
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Imagine a school principal assigns students to classrooms based on behavior, placing students who need stronger discipline into one class and highly motivated students into another. If a researcher uses these pre-existing classes to test a new educational program without random assignment, apply the concept of confounding variables to explain in one to three sentences how this assignment method threatens the internal validity of the study.