3_19 - Experimental Research True Experiments Random...

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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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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. Giving pretest can be a threat to the internal validity.
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3_19 - Experimental Research True Experiments Random...

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