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Imagine you are reviewing a research proposal that intends to use a nonequivalent groups design to compare a new therapy against a standard therapy using two separate, pre-existing support groups. In one to three sentences, identify the specific threat to the study's internal validity that must be addressed and state what causes this threat.

Question: Imagine you are reviewing a research proposal that intends to use a nonequivalent groups design to compare a new therapy against a standard therapy using two separate, pre-existing support groups. In one to three sentences, identify the specific threat to the study's internal validity that must be addressed and state what causes this threat.

Sample answer: The specific threat to internal validity is the introduction of potential confounding variables due to the comparison groups being nonequivalent. This is caused by the lack of random assignment, which means the two pre-existing support groups likely have inherent, initial dissimilarities before the therapy even begins.

Key points:

  • The primary threat to internal validity is the presence of potential confounding variables.
  • This threat is caused by the inherent, pre-existing differences between the two support groups.
  • These differences arise because the researcher cannot use random assignment to allocate participants to the therapy conditions.

Rubric: Award credit if the student identifies confounding variables (or pre-existing differences) as the threat to internal validity and attributes this threat to the lack of random assignment in the pre-existing groups.

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Updated 2026-05-27

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Research Methods in Psychology - 4th American Edition @ KPU

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