experimental designs

• types of experimental design

  • independent group design (igd)
  • matched pairs design (mp)
  • repeated measures (rm)
• what is independent group design?

  • randomly put into groups (e.g number tables) - referred to as lottery method
  • group participates in one condition of the iv
• matched pairs design

  • pretest (e.g IQ) to match participants in each group (rank the participants, then go down list and put one in each group)
  • participants take part in one condition of IV
• what are repeated measures?

  • everyone does condition a and condition b of IV (both conditions)
• repeated measures counterbalancing

  • solves problem of order effect
  • everyone completes both conditions, but the starting order is different (e.g group 1 start on condition a, then do b. group 2 start on condition b, then do a.)
• what is order effect

  • for repeated measures, the order of completing conditions produces an unwanted result.
  • this is a confounding variable
• disadvantages of independent group design

  • expensive and time consuming- larger population required (x2)
  • biased towards a group- random allocation
• advantages of matched pairs

  • no problems with order effects, less likely to produce demand characteristics
  • less bias- groups are split evenly
• disadvantages of matched pairs

  • expensive- larger sample required and pretests
  • time consuming + possibility of participant variables affecting the outcome, as participants can't always be matched identically
• advantages of repeated measures

  • no bias- both groups are identical, so paricipant variables as an extraneous variable are controlled, meaning removed.
  • smaller sample can be used- becomes cheaper and quicker
• disadvantages of repeated measures

  • order effect- could become tired, bored or gain practice for the test style (must be remedied by counterbalencing). this increases the chance of demand characteristics.
• what can repeated measures be used for?

  drug tests
• what can independent group design be used for

  deception
• advantages of independent group design

  • no problems with order effects, less likely to experience demand characteristics- removing participant variables (extraneous varibles controlled) meaning the validity is higher.
  • easier compared to matched pairs