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Quasi-Experimental Designs

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 NE and QE compare scores from diff groups or conditions but there is no manipulated variable. Quasi.

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Quasi-Experimental Designs

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  1. 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. Quasi • “having some but not all of the features” • Examples: lack of control or comparison group, one group give treatment and then assessed • No true IV • Participants already part of a group with preexisting conditions

  4. Nonmanipulated IV • Participants not randomly assigned to conditions but come as members of a condition • Subject (participant) variable • Gender • Age

  5. Single-Group Posttest-Only Design • Single group of participants given a treatment then tested • No comparison group or previous measurements to compare • Cannot be used to draw conclusions about how an experiences has affected P’s

  6. Single-GroupPretest/Posttest Design • Measurements taken before treatment and after treatment. • Differences compared and any changes assumed to be treatment • Problem is still no comparison group

  7. Single-Group Time-Series Design • Single group of participants measured repeatedly before and after a treatment. • Allows for better baseline before treatment and continued measurement after treatment

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

  9. 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

  10. NE Control Group Posttest-Only Design • Two nonequivalent groups (NE) given treatment and then posttest measure • NE 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

  11. NE Control Group Pretest/Posttest Design • At least two NE groups given a pretest, then treatment, then posttest • Still no random assignment • Treatment only given to Experimental group • Compare between groups and within groups

  12. 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

  13. 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

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

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