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lec10 - Chapter 10 Quasi-Experimental and Single-Case...

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Chapter 10 Quasi-Experimental and Single-Case Designs (Reminder: Don’t forget to utilize the concept maps and study questions as you study this and the other chapters.) The experimental research designs discussed in this chapter are used when it is impossible to randomly assign participants to comparison groups (quasi-experimental designs) and when a researcher is faced with a situation where only one or two participants can participate in the research study (single case designs). Like the designs in the last chapter, quasi-experimental and single-case designs do have manipulation of the independent variable (otherwise they would not be “experimental research” designs). Quasi-Experimental Research Designs These are designs that are used when it is not possible to control for all potentially confounding variables; in most cases this is because the participants cannot be randomly assigned to the groups. Causal explanations can be made when using quasi-experimental designs but only when you collect data that demonstrate that plausible rival explanations are unlikely, and the evidence will still not be as strong as with one of the strong designs discussed in the last chapter. You can view quasi-experiments as falling in the center of a continuum with weak experimental designs on the far left side and strong experimental designs on the far right side. (In other words, quasi designs are not the worst and they are not the best. They are in-between or moderately strong designs.) /------------------------------------/------------------------------------/ Weak Quasi Strong Designs Designs Designs Three quasi-experimental research designs are presented in the text: the nonequivalent comparison-group design, the interrupted time-series design, and the regression discontinuity design. Nonequivalent Comparison-Group Design This is a design that contains a treatment group and a nonequivalent untreated comparison group about of which are administered pretest and posttest measures. The groups are “nonequivalent” because you lack random assignment (although there are some control techniques that can help make the groups similar such as matching and statistical control). Because of the lack of random assignment, there is no assurance that
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the groups are highly are similar at the outset of the study. Here is a depiction of the nonequivalent comparison-group design: Because there is no random assignment to groups, confounding variables (rather than the independent variable) may explain any difference observed between the experimental and control groups. The most common threat to the internal validity of this type of design is differential selection. The problem is that the groups may be different on many variables that are also related to the dependent variable (e.g., age, gender, IQ, reading ability, attitude, etc.).
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