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Nonexperimental and Quasi-Experimental Designs

Nonexperimental and Quasi-Experimental Designs. Distinction is the degree of control over internal validity. True Experiment. Manipulation of a variable Control of threats posed by confounding variables

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Nonexperimental and Quasi-Experimental Designs

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  1. Nonexperimental and Quasi-Experimental Designs Distinction is the degree of control over internal validity

  2. True Experiment • Manipulation of a variable • Control of threats posed by confounding variables • NE and QE compare scores from diff groups or conditions but there is no manipulated variable

  3. Nonequivalent Group Designs • Groups / conditions created by differences in individuals before experiment • No control over assignment of individuals to groups • Examples: IQ, race, gender

  4. Differential Research Design • NE research design because no attempt to control for threat of assignment bias • Researcher interested in differences between groups on certain variable • Ex post factolooks at differences after the fact

  5. NE Control Group Design • Nonequivalent groups used in applied research settings in which goal is to evaluate treatment to a preexisting group of ind • Second, control group, used as comparison

  6. Posttest-only nonequivalent control group design • “static group comparison” • One group of p’s given treatment and then measured after treatment (posttest). Scores then compared to control group X 0 (treatment group) 0 (control group)

  7. Caution! • Remember the problem with threat to internal validity with these designs • Assignment bias differences between groups could reflect individual differences

  8. Pretest-Posttest NE Control Group Design • Phase 1 • Observe (measure) both groups • Phase 2 • Administer treatment to Experimental group • No treatment to control Group 0 X 0 (Experimental group) 0 0 (control group) • Purpose of Phase 1 to determine if groups equal prior to Phase 2 treatment • Quasi-Experimentalthreats minimized

  9. Threats still possible • Differential effects • Differences between groups may be result of different histories effects • Similar to influences such as maturation, instrumentation, testing effects etc

  10. Pre-Post design • One group of Ps measured before and after a treatment • Goal is to evaluate influence of intervening treatment • No control group • Series of measurements over time on same Ps • Caution: time-related threats

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