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Quasi-experimental Designs



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Quasi experimental Designs Cozby Ch 11 Intro to Experimental Psychology Psych 185 Dr Isonio Golden West College Recap Designs To date we have discussed True experimental designs Chapters 8 and 10 What remains is today s topic Quasi experimental designs These are intermediate designs they lack the randomization and full control of extraneous variables that true experiments have but can nevertheless yield reasonably reliable conclusions For completeness we will also mention a couple of non experimental designs what Campbell and Stanley call pre experiments that are fraught with confounds Internal Validity Remember Internal validity refers to the extent to which we are able to clearly attribute the observed differences on the dependent variable to the effects of the independent variable Thus true experimental designs have high internal validity and quasi experimental designs have some intermediate level of internal validity Threats to Internal Validity Factors which if uncontrolled can confound i e mess up a study The classic list comes from Campbell and Stanley Threats to Internal Validity History Maturation Repeated Testing Instrument Decay Statistical Regression Selection Mortality Threats to Internal Validity History Outside events beyond the context of the study itself influence the participants during the study possible affecting their behavior This threat is relevant in any type of repeated measures design Pretest IV Posttest Threats History Example of History operating as an internal validity threat Study to assess how attitudes toward welfare program are affected by having a personal discussion with recipients Pretest measure of attitudes on Monday discussion occur Tuesday through Thursday posttest measure of attitudes on Friday BUT a special TV documentary reporting on widespread welfare fraud was aired on Wednesday evening Results show no difference between pre and posttest attitude assessments due to true lack of difference or the confounding effect of the



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