3_19 - Experimental Research True Experiments Random...

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1 Experimental Research True Experiments Random assignment of subjects to groups Random assignment makes groups equivalent (hopefully) in all variables, including prior subject characteristics. Since the random assignment eliminates the selection threat of internal validity, the design is considered that the design has strong internal validity in general . There are still other possible threats to internal validity; mortality, Hawthorn effect, instrumentation, etc. Quasi-Experiments Non-random assignments of subjects Weaker internal validity Æ need efforts to make the design internally valid
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2 True Experimental Designs 1. Randomized posttest-only two-group design R X1 O R X2 O Random assignment ensures the two groups are equivalent. However, there is still a chance that the performance levels between the two groups are different before the experiment. 2. Randomized pretest-posttest two-group design R O X1 O R O X2 O This design can check if the performance levels between the groups are really similar before the experiment.
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