Experimental and Control Groups & Within-Participant Designs

EXPERIMENTAL AND CONTROL GROUPS

  • Experiments can involve 11 or more experimental groups and 11 or more control groups.
  • The researcher manipulates the independent variable to create these groups.
  • Experimental group: participants who receive the treatment that is of interest to the researcher, or a particular drug under study — that is, the participants who are exposed to the change that the independent variable represents.
  • Control group: as much like the experimental group as possible and treated in every way like the experimental group except for that change. The control group provides a comparison against which the researcher can test the effects of the independent variable.
  • Examples:
    • Smiling study: experimental group interacts with a confederate who smiles a lot; control group may have the confederate with a blank expression, frown, or smile just a little.
    • Meaning in life study: participants who listened to happy music (experimental) vs those who heard neutral music (control).
  • Within-participant design (to keep groups as similar as possible): the same group of participants experiences the various conditions, effectively serving as their own control group.
  • Random assignment (to produce equivalent groups) ensures that the groups are comparable at the start; the same group of participants experiences the various conditions.

Within-Participant Designs

  • Definition: Within-Participant Design — participants serve as their own control group.
  • Implementation: the same group of participants experiences the various conditions in the study.
  • Purpose: reduces between-subject variability and helps ensure equivalence between conditions by using the same participants across conditions.