18: Internal Validity: Subject Selection

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Internal Validity

  • Subject selection is an important component of internal validity
  • If the subjects differ before research begins, can we truly say a difference at the end of the study is significant?
  • In order to make sure subjects are the same at the beginning of the study, several methods can be applied:
    1. Random selection and random assignment
    2. Subject matching
    3. Pretest-posttest