Experimental Psychology (Chapters 9,11, & 12)

• Experimental Design


  A basic plan or design for the experiment; the general structure of the experiment
• Between-Subjects Design


  • Different subjects take part in each condition of the experiment
  • We draw conclusions from between-subjects experiments by making comparisons between the behaviors of different groups of subjects
  • A subject participates in only one condition of the experiment
• The more the sample resembles the whole population, the more likely it is that the behavior of the sample mirrors that of the whole population
• Do not use your friends
• Encourage your subjects
• The representativeness of our sample determines whether we can generalize our results to the entire population
• Random Sampling increases an experiment's external validity
• How Many Subjects?


  • 20-30 subjects in each treatment condition to detect a strong treatment effect
  • Fewer subjects in each condition risks not detecting the effect
• Effect size


  • A statistical estimate of the size or magnitude of the 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
• Two-group design


  • When only two treatment conditions are needed, the experimenter may choose to form two separate groups of subjects
• Two-independent-groups design


  • Subjects are placed in each of two treatment conditions through random assignment
  • Used when one IV must be tested at two treatment levels or values
• Random Assignment


  Every subject has an equal chance of being placed in any of the treatment conditions
• Experimental Condition


  We apply a particular value of our independent variable to the subjects and measure the dependent variable
• Experimental Group


  The subjects in an experimental condition
• Control Condition


  Used to determine the value of the dependent variable without an experimental manipulation of the independent variable
• Control Group


  The subjects in a control condition
• Placebo Group


  Control condition in which subjects are treated exactly the same as subjects who are in the experimental group, except for the presence of actual drug
• Two-Experimental-Groups Design


  • Can be used to look at behavioral differences that occur when subjects are exposed two different values or levels of the IV
  • Includes the experimental group-control group
• Two-Matched-Groups Design


  • There are two groups of subjects, but the researcher assigns them to groups by matching or equating them on a characteristic that will probably affect the dependent variable
• Precision Matching


  We insist that the members of the matched pairs have identical scores
• Range Matching


  We require that the members of a pair fall within a previously specified range of scores
• Rank-Ordered Matching


  The subjects are simply rank ordered by their scores on the matching variable, and subjects with adjacent scores then become a matched pair
• Matching procedures are especially useful when we have very small numbers of subjects because there is a greater chance that randomization will produce groups that are dissimilar
• The larger the treatment groups, the better the chances are that randomization will lead to similar groups of subjects and the less need there may be for matching
• When we match, it is essential that we match on the basis of an extraneous variable that is highly related to the dependent variable of the experiment
• Multiple-Groups Design


  • A design in which there are more than two groups of subjects and each group is run through a different treatment conditions
• Multiple-Independent-Groups Design


  • The subjects are assigned to the different treatment conditions at random
• 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
• Pilot Study


  • A mini-experiment in which treatments are tested on a few subjects to see whether the levels seem to be appropriate or not
  • A good way to work out any bugs in the procedures of an experiment before the real experiment is underway
  • Allows you to make changes before you invest the time and resources in a large-scale experiment