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Presentation 6 Between- vs. Within-Subjects Designs (Click on the left mouse button to continue. Click on the right mouse button and select “Previous” to go back.)
Quick Review Psychology is the scientific study of human behavior. To study human behavior, psychologists rely on the scientific method. Use of the scientific method results in the development of study hypotheses. An experiment is designed to test those study hypotheses. The results from these tests either support or fail to support the null hypothesis.
A True Experiment There are many ways to conduct an experiment. However, there is only one TRUE experiment. A True Experiment contains: – An Independent Variable (or IV) that is manipulated by the experimenter – A Dependent Variable (or DV) that is observed and recorded by the experimenter
True Experiments (cont.) To manipulate or control the IV, an experimenter sets the value of the IV to at least two different levels during the course of the experiment. For example, administering three different amounts of a drug (three levels of the IV) to measure how much the frequency of side effects (the DV) increases with dosage.
Two Types of Experimental Design To run an experiment, the experimenter has two general types of experimental design to choose from: 1) Between-Subjects Design 2) Within-Subjects Design
Between-Subjects Design A Between-Subjects Design is used when a different group of subjects is assigned to each level of the IV. There are two types of Between-Subjects Designs: 1) Randomized Group and 2) Matched Group.
Randomized Group Designs Randomly assign your subjects to each level of your IV. The number of groups in your experiment should equal the number of levels of your IV.
Randomized Group Design (cont.) Advantages: Simple to carry out (small groups of subjects needed). No pre-testing or categorization of subjects is needed. The statistical analysis is straight-forward.
Randomized Group Design (cont.) Disadvantages: Could provide limited info on the effect of the IV on the DV, particularly if there are only 2 or 3 levels of the IV. May not be sensitive to the effect of the IV. Subjects may vary wildly on other characteristics that influence the DV. This makes it difficult to test the influence of the IV on the DV.
Matched Groups Design Subjects are matched by a set of characteristics that may influence the DV. Matched subjects are then distributed evenly across each level of the IV. This design attempts to control for subject characteristics that influence the DV. Thus, it may be more likely that the influence of the IV on the DV will be observed.
Matched Groups Design (cont.) Advantages: Allows you to control for nuisance variables that may otherwise obscure the effect of the IV on the DV. Disadvantages: Requires pre-testing and matching of subjects. Critical that the measures you use to match subjects are reliable and valid.
Recap - Between Subjects Designs Involves the assignment of subjects to each level of the IV. Randomized Groups Designs are relatively simple to carry out, but the influence of the IV on the DV may be less likely to be observed. Matched Groups Designs are more likely to observe the influence of the IV on the DV, but this design is more difficult to carry out.
Within-Subject Designs An alternative to the Between-Subjects design. Each subject is exposed to all levels of the IV, rather than being assigned to just one level. Conceptually like the Matched-Group Between Subjects Design in that the individual is “matched” with themselves across each level of the IV.
Within-Subjects Designs (cont.) Advantages Subjects are “matched” across all levels of the IV. This design controls the error variance due to extraneous variables (e.g., controls for subject characteristics that influence the DV).
Within-Subjects Designs (cont.) Disadvantages More demanding on subjects (e.g., time and resources) Carryover effects - when one treatment effects the results of a future treatment.
Examples of Carryover Effects Learning - the subject learns something in one treatment that affects another.
Carryover Effects (cont.) Learning Fatigue - the subject gets tired or bored.
Carryover Effects (cont.) Learning Fatigue Habituation - the subject gets too comfortable or relaxed - reduced responsiveness to stimuli.
Carryover Effects (cont.) Learning Fatigue Habituation Sensitization - the subject becomes overly responsive to stimuli.
Carryover Effects (cont.) Learning Fatigue Habituation Sensitization Contrast - the subject attempts to compare the conditions of the experiment.
Carryover Effects (cont.) Learning Fatigue Habituation Sensitization Contrast Adaptation - builds a natural tolerance to the stimuli.
Use the Within-Subjects Design When: The characteristics of the subjects may influence the DV and are difficult to control. A large number of subjects is not available. The researcher is assessing the effect of increasing levels of an IV on a DV.
Summary Psychologists conduct experiments to test hypotheses. When conducting experiments, researchers can use a Between-Subjects design, a Within- Subjects design, or a combination of both (called a mixed-design). The Between-Subjects design is more simple to conduct than the Within-Subjects design, but the Within-Subjects design may be better able to demonstrate the effect of the IV on the DV.
Disclaimer & Credits These PowerPoint Presentations were created to supplement the materials presented in Psychology 202Q. They were not intended to replace attendance in lecture or lab. If any material in these presentations conflicts with what your instructor covers, always go with what s/he says. The Psychology 202Q PowerPoint Presentations were created by Steven Rumery in 1998, and updated by Poom Nukulkij in Fall 2000. For additional information, check out the Psychology 202Q Virtual Classroom at http://www.sp.uconn.edu/~ps202vc.