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7.5: Between Subject Designs

  • Page ID
    127257
    • Anonymous
    • LibreTexts
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    Learning Objectives
    • Identify the core features of between subject experimental designs.
    • Differentiate between the different types of between subject designs.

    What is a Between Subject Design?

    The simplest true experimental designs are two group designs involving one treatment group and one control group, and are ideally suited for testing the effects of a single independent variable that can be manipulated as a treatment. These designs are often depicted using a standardized design notation, where R represents random assignment of subjects to groups (using either randomization or matching), X represents the stimulus administered to the experimental group, and O represents pretest or posttest observations of the dependent variable (with different subscripts to distinguish between pretest and posttest observations of treatment and control groups).

    This type of design does include the classical experimental design as well. This design is called the pretest-posttest control group design.

    Types

    Pretest-posttest control group design (classical experiment design): In this design, subjects are randomly assigned, using either matching or randomization, to experimental and control groups, subjected to an initial (pretest) measurement of the dependent variables of interest, the experimental group is administered a stimulus (representing the independent variable of interest), and the dependent variables measured again (posttest). The notation of this design is shown below:

    R 01 X 02  R 01 02

    The top group is the experimental group and the bottom group is the control group.

    The effect of the stimulus in the pretest posttest design is measured as the difference in the posttest and pretest scores between the treatment and control groups.

    The pretest posttest design handles several threats to internal validity, such as maturation, testing, and regression, since these threats can be expected to influence both treatment and control groups in a similar (random) manner. The selection threat is controlled via random assignment. However, additional threats to internal validity may exist. For instance, mortality can be a problem if there are differential dropout rates between the two groups, and the pretest measurement may bias the posttest measurement (especially if the pretest introduces unusual topics or content).

    Posttest-only control group design: This design is a simpler version of the pretest-posttest design where pretest measurements are omitted. The design notation is shown below.

    R X 02  R  02

    The top group is the experimental group and the bottom group is the control group.

    The stimulus effect is measured simply as the difference in the posttest scores between the two groups:

    This design controls for maturation, testing, regression, selection, and pretest-posttest interaction, though the mortality threat may continue to exist.

    The Solomon four-group design: This design is the most complex and it combines both the previous designs into a single one. So in this design there are essentially 4 groups. Two experimental groups and two control groups.

    R 01 02   R 01  01 R 01 X 02  R 01 02

    The and second groups are experimental groups and the second and fourth groups are control groups. Please note the same stimulus is given to all of the experimental groups.

    The stimulus effect is measured simply as the difference in the posttest scores between the control and experimental groups.

    The major advantage of this type is it controls for all the threats to internal validity the others ones have. However, this can be very expensive to use as well as time consuming.

    KEY TAKEAWAYS
    • These designs allow for group comparisons.
    • Each between subject design has strengths and weaknesses.

    This page titled 7.5: Between Subject Designs is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by Anonymous.