chapter notes PSYC_3150_10
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This 2 page Class Notes was uploaded by Gabriela Saint-Louis on Friday March 20, 2015. The Class Notes belongs to PSYC_3150_10 at George Washington University taught by Forssell in Spring2015. Since its upload, it has received 112 views. For similar materials see Psychology of Sex Differences in Psychlogy at George Washington University.
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Date Created: 03/20/15
Chapter 8 Experimental Design Confounding and internal validitv Confounding variable is a variable that varies along with the independent variable confounding occurs when the effects of the independent variable and an uncontrolled variable are intertwined so you cannot determine which of the variables is responsible for the observed effect good experimental design requires eliminating possible confounding variables that could result in alternative explanations Researcher can claim that the independent variable caused the results only by eliminating competing alternative explanations Internal Validity when the results of an experiment can con dently be attributed to the effect of the independent variable Basic Experiments variable and the dependent variable the basic simple experimental design cn take one of two forms a posttest only design or a pretestposttest design the simplest possible experimental design as two variables independent Posttestonlv desina three steps in a posttestonly design 0 obtain two equivalent groups of participants Groups must be equivalent to avoid potential selection differences cannot differ in any systematic way Groups can be made equivalent by randomly assigning participants to the two conditions or by having the same participants participate in both conditions 0 introduce the independent variable amp Researcher must choose two levels of the independent variable such as an experimental group that receives a treatment and a control group that does not Ex a study testing the effect of a treatment method for reducing smoking could compare a group that receives the treatment with a control group that does not 0 measure the effect of the independent variable on the dependent variable the same measurement procedure is used for both groups so that comparison of the two groups is possible because the groups were equivalent prior to the introduction of the independent variable and there were no confounding variables any difference between the groups on the dependent variable must be attributed to the effect of the independent variable Pretestposttest onIv design the only difference between the posttestonly design and the pretest posttest design is that in the latter a pretest is given before the experimental manipulation is introduced in this design researcher can be sure that the groups were in fact equivalent at the beginning of the experiment not usually necessary if the participants have been randomly assigned to the two groups 0 with a large sample of participants random assignment will produce groups that are virtually identical in all respects 0 the larger the sample the less likelihood there is that the groups will differ in any systematic way prior to the manipulation of the independent variable comparing posttestonly and pretestposttest Designs a pretest enables the researcher to assess whether the groups are in fact equivalent to begin with sometimes a pretest is necessary to select the participants in the experiment the pretestposttest desing immediately makes us focus on the change from pretest toposttest a pretest is alsonecessarywhenever there is a possibility that participants will drop out of the experiment
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