8.3: Limiting Confounding Individual Differences

• Random Assignment

  A random process is used to assign participants to groups; thus, we minimise the potential for confounding from individual differences because it is unlikely that any group is systematically "better."
• Restricted Random Assignment

  The group assignment process is limited to ensure predetermined characteristics (such as equal size) for the separate groups.
• Matching

  Involves assigning individuals to groups so that a specific participant variable is balanced, or matched, across the groups. The intent is to create groups that are equivalent with respect to the variable matched.

  1. Identification of the variable(s) to be matched across groups.
  2. Measurement of the matching variable for each participant.
  3. Assignment of participants to groups by means of a restricted random assignment that ensures a balance between groups.
• Holding a Variable

  When it is suspected or found that a particular confounding variable may be creating major differences between the groups, the solution is to eliminate said confounding variable altogether.
• Restricting Range

  When it is suspected or found that a particular confounding variable may be creating major differences between the groups, the solution is to narrow the effects of said confounding variable within an appropriate range that can still facilitate valid results.
• What is the serious drawback behind the methods: "holding a variable" and "restricting range?"

  Whenever a variable is prevented from reaching its natural range of variation, the external validity of the research is limited; thus, it is difficult to generalise this to the whole population.