PSY 3213L Lecture 8: Chapter 8 Experimental Designs

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Clinical and Health Psychology
PSY 3213L
Melissa Cervantez

Chapter 8: Experimental Designs Thursday, July 13, 2017 2:37 PM • Experimental Method o Researcher attempts to control all the extraneous variables o Researcher manipulates the IV to create groups and compares the groups on the DV • All other variables are kept constant ▪ Experimental control or randomization • Confounding Variable o Variable that varies along with the IV o Cannot determine which of the variables is responsible for the observed effect • Good experimental Design o Eleiminates confounding variables o Designed so that the IV can be the cause fo the results • Has internal validity • Basic Experiments o Simplest design has two variables • IV and DV o The IV has 2 levels • Experimental and control group o Posttest only design or a pretest-posttest design • Posttest-Only Design o Must obtain two equivalent groups of participants • Make sure there are no selection differences • Randomly assign to groups • Participate in both conditions o Introduce the IV o Measure effect of IV on DV • Pretest-Posttest Design o Allows to compare the before and after effect o Confirm if groups were equivalent • When to use Pretest o If you have a large enough sample your randomization should make your groups equivalent though o If you have a small group, then you should use pretest • Tests are important for pretest and posttest and must be incorporated in analysis • Comparing Pretest and Posttest o A pretest is necessary whenever there is a possibility that participants will drop out of the experiment; this is most likely to occur in a study that lasts over a long time period. The dropout factor in experiments is called Attrition or mortality o One disadvantage of a pretest, however, is that it may be time-consuming and awkward to administer in the context of the particular experimental procedures being used. Perhaps most important, a pretest can sensitize participants to what one is studying, enabling them to figure out what is being studied and potentially why o It is also possible to assess the impact of the pretest
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