PSY 290A Lecture Notes - Lecture 30: Demand Characteristics, Internal Validity, Observer-Expectancy Effect

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The really bad experiment (aka the one-group pretest/posttest design) Six potential internal validity threats in one-group, pretest/posttest designs. If needed, rerun the study with a stronger manipulation. Ceiling or floor effects (on dv or iv) Use a manipulation check for iv: within-groups variability obscured the group differences, measurement error. Use a precise measure: measure more often. Responsible experimenters may conduct double-blind studies, use computers to measure variables, or put people in contrived, controlled environments to eliminate internal validity threats and to increase a study"s power to avoid null effect. If the study passes all your internal validity queries, you can conclude with confidence that the study was a strong one: you can trust the result and make a causal claim. If you encounter a study, in which the independent variable had no effect on the dependent variable (a null effect), you can review the possible obscuring factors: obscuring factors can be sorted into two categories of problems.

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