Experimental Design

Good experimental design
1. Confounding variables
  1. Cannot determine whether an independent variable and an uncontrolled variable is responsible
2. Internal validity
  1. Extent to which the results can be attributed to the independent variable
Basic experiment
1. An independent variable with two levels
  1. Experimental and control group
Independent groups design
1. Also called between groups design
2. Different participants are assigned to each level of the independent variable
3. Pre post test design
  1. Pre test scores are compared to check that both groups are equal in the critical variable
  2. Pre test can be used to:
    1. Determine highest and lowest scores
    2. For small sample sizes
    3. In longitudinal studies
4. Solomon four group design
  1. Treats the pretest as a second independent variable
  2. 1/2 receives only the dependant variable, others receive the pre/post test
5. Repeated measures design
  1. Also the within subjects design
  2. The same participants participate in both conditions
  3. Less error variance b/c participants do not vary
  4. Effects that may occur:
    1. Order effect
      1. Order of presenting the treatments affects the dependent variable
      2. Counterbalancing
        Complete counterbalancing
        All possible orders of presentation are included
        Counteracts order effect
        Partial counterbalancing
        Uses latin square
        Each condition appears at each ordinal position and precedes and follows each condition
    2. Practice effect
      1. Improvement of performance as a result of practice
    3. Fatigue effect
      1. Deterioration in performance as the participant becomes tired
      2. Time interval
        Rest period can counteract a fatigue effect.
    4. Contrast effect
      1. Response to the second condition is alter b/c the 2 conditions are contrasted
Matched pairs design
1. Match people on a participant characteristic
2. Ensures that groups are equal

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Experimental Design

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