EXPERIMENTAL PSYCHOLOGY

• Experimental design


  The plan for testing a hypothesis, the experiment's structure or floor plan, not the specific content
• We can use the same design to investigate different hypotheses
• Factors that determine an experimental design

  • The number of independent variables in the hypothesis
  • The number of treatment conditions needed to fairly test
  • Whether the same subjects are used in each of the treatment conditions
• Between-subjects design


  A subject participates in only one condition of the experiment
• Generalizability


  The representatives of our sample determine whether we can generalize our results to the entire population from which the sample was drawn
• Random sampling


  Increases an experiment's external validity
• Minimum number of subjects per group

  10 to 20 subjects in each treatment condition to detect a strong treatment effect
• Effect size


  A statistical estimate of the size or magnitude of a treatment effect, the larger the effect size the stronger the relationship between the independent and dependent variables and the fewer subjects needed to detect a treatment effect
• Effect size


  Determines the number of subjects required to detect a treatment effect
• Two-group design


  Involves the creation of two separate groups of subjects
• Two-independent groups design


  One IV with two levels and subjects are randomly assigned to one of the two conditions
• Random assignment


  Assigning subjects to conditions so that each subject has an equal chance of participating in each condition, to equally distribute subject variables between the treatment groups to prevent them from confounding an experiment
• Experimental condition


  Presents a value of the independent variable
• Control condition


  Presents a zero level of the independent variable
• Two experimental groups design

  Assign subjects to one of the two levels of the independent variable
• Random assignment works poorly with 5 to 10 subjects per condition, and may not control all extraneous variables
• Two matched group design

  Match participants on a subject variable correlated with the DV, and randomly assign them to one of two treatment conditions
• Matching


  Used to create groups that are equivalent on potentially confounding subject variables, to prevent selection threat from undermining internal validity
• Precision matching


  Form pairs of identical IQ scores, randomly assign members of each pair to one of two treatment conditions
• Range matching


  Form pairs of scores that fall within a specified range, randomly assign members of each pair to one of two treatment conditions
• Rank ordered matching


  Rank all IQ scores from highest to lowest, form pairs of scores that fall in adjacent tracks, randomly assign members of each pair to one of two treatment groups
• Two matched group designs should be used when there are two levels of an independent variable and there is an extraneous variable we can measure that could affect the dependent variable
• Multiple groups design


  A between subjects design with more than two levels of an independent variable
• Multiple independent groups design


  Randomly assign subjects to one of the treatment conditions
• Block randomization


  A process for randomly assigning equal numbers of subjects to conditions, the experimenter creates random sequences of each experimental condition and subjects are randomly assigned to fill each treatment block
• The hypothesis, prior research, pilot study results, and practical limits can all help determine the number of treatments
• Pilot study


  A trial run of the experiment that uses a few subjects, can help the experimenter refine the procedure or determine whether the experiment is promising