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QUASI-EXPERIMENTS w Compare subjects in different conditions on a DV w Lacks one or more criteria for an experiment (cause, comparison, control) w Interpreted like a correlational study
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Why Do Quasi-Experiments? w Internal validity is a problem because the IV is not manipulated w It may not be possible or ethical to manipulate the IV
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Ex Post Facto Designs w Compare groups that differ on a pre- existing variable (subject variable) w Examples: gender personality presence of mental disorder
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One-Group Posttest Only Design w Scores are measured after a treatment for one group w The treatment is not manipulated, and there is no comparison
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One-Group Pretest-Posttest Design w Scores are measured before and after an event or treatment w The treatment is not manipulated
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Non-Equivalent Control Group Design w Compare a treatment group and a control group before and after the treatment w The groups are not randomly assigned
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Single Group Time Series Design w Scores are measured several times before and after an event or treatment w Better than Pretest-Posttest design, because you can tell whether the change is likely to be due to a random fluctuation
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Multiple Time Series Design w Scores for a treatment and control group are measured several times before and after w The control group is not randomly assigned w Combination of Nonequivalent Control Group and Interrupted Time Series
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Cross-Sectional Design w Compare different age groups at one point in time w The groups may differ on other variables, since age is not manipulated w cohort effects - generational history differences
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Longitudinal Design w Measure the same group at different ages w Time-consuming w Attrition can be a problem
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Cohort Design w Compare different age groups at different ages w Combination of longitudinal and cross- sectional
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Statistics for Quasi-Experiments w Choose from the same set of statistics used for experiments w Remember that you interpret differences like the study was correlational w Take into account number of conditions and whether variables are between or within subjects
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