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Experimental Designs

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EXPERIMENTAL DESIGNS FOR RESEARCH
Causality
Experimental Designs
Control Group Pre-test/Post-test Design
Threats to Internal Validity
Threats to External Validity
Post-Test only Control Group Design CAUSALITY To establish whether two variables are causally related, that is, whether a change in the independent variable X results in a change in the dependent variable Y, you must establish: 1) time order--The cause must have occurred before the effect; 2) co-variation (statistical association)-- Changes in the value of the independent variable must be accompanied by changes in the value of the dependent variable; 3) rationale-- There must be a logical and compelling explanation for why these two variables are related; 4) non-spuriousness-- It must be established that the independent variable X, and only X, was the cause of changes in the dependent variable Y; rival explanations must be ruled out. To establish causality, one must use an experimental or quasi-experimental design. Note that it is never possible to prove causality, but only to show to what degree it is probable. EXPERIMENTAL DESIGNS True experimental designs include: Pre-test/Post-test control group design Solomon Four-Group design Post-test only control group design

Pre-test/Post-test control group design
This is also called the classic controlled experimental design, and the randomized pre-test/post-test design because it: 1) Controls the assignment of subjects to experimental (treatment) and control groups through the use of a table of random numbers. This procedure guarantees that all subjects have the same change of being in the experimental or control group. Because of strict random assignment of subjects, it is assumed that the two groups are equivalent on all important dimensions and that there are no systematic differences between the two

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