Problem of Systematic Differences in Experiments
Systematic differences are preexisting variations between experimental and control groups before an experiment begins, such as one group being composed entirely of a specific gender, ethnic identity, or religious ideology. If groups are systematically different, researchers cannot determine whether any observed differences in the final outcomes were caused by the manipulation of the independent variable or by these preexisting traits.
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Logic of Causal Inference in Controlled Experiments
A researcher wants to test the effectiveness of a new public speaking training program. They allow the first 40 volunteers who sign up to participate in the new program (the experimental group). The next 40 volunteers are assigned to a waitlist and will serve as the control group. The researcher finds that the experimental group performs significantly better on a final speaking task. Why is it difficult to conclude that the training program caused the improvement?
Problem of Systematic Differences in Experiments
What is the primary purpose of using random assignment in an experimental design?
In an experiment with 200 participants, a researcher uses random assignment to place participants into either a meditation group or a control group. A colleague argues that random assignment guarantees the two groups will have identical average ages, stress levels, and personality traits before the study begins. Is this claim accurate?