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PSY309H1 (11)
Chapter 8

PSY309 - Chapter 8 Notes

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University of Toronto St. George
John Kloppenbord

Chapter 8: Experimental Design Confounding and Internal Validity - the researcher manipulates the independent variable to create groups that differ in the levels of the variable and then compares the groups in terms of their scores on the dependent variable, while other variables are kept constant through: experimental control or randomization - Confounding Variable – a variable that varies along with the independent variable and therefore not controlled - confounding occurs when the effects of the independent variable and an uncontrolled variable are intertwined so the variables responsible for the observed effect cannot be determined - good experimental design involves eliminating possible confounding that results in alternate explanations - Internal Validity – the certainty with which results of an experiment can be attributed to the manipulation of the independent variable rather than to some other, confounding variable - to achieve internal validity, the researchers must design and conduct the experiment so that only the independent variable can be the cause of the results Basic Experiments - the simplest experimental design has two variables: independent variables and dependent variables - the independent variable has two levels: experimental group and control group - the simplest experimental design can take two forms: posttest-only design or pretest-posttest design - Posttest-Only Design  dependent variable is measured only once, AFTER manipulation of the independent variable 1.Choose participants and assign them to two equivalent groups - Selection Differences – the ppl. selected to be in the conditions cannot differ in any systematic way 2.Choose levels of the independent variable (ie. experimental group receiving treatment and control group that doesn’t) 3.Measure the effect of the independent variable on the dependent variable - b/c the groups were equal to begin to with, there should be no confounding variables; any difference b/w the groups on the dependent variable must be attributed to the effect of the independent variable - Posttest-Pretest Design dependent variable is measured BEFORE and AFTER manipulation of the independent variable - ensures that the groups were equal at the beginning of experiment - the larger the sample, the less likely that the groups will differ prior to the manipulation of the independent variable - but it is more likely that any difference b/w groups on the dependent variable is due to the effect of the independent variable - to detect a statistically significant effect, you need a minimum of 20-30 participants per condition - Advantages of a PRETEST o enables the researcher to assess whether the groups were equal to begin with o Necessary to select the participants in the experiment o Can measure the extent of change in each individual o Necessary whenever there is a possibility that participants will drop out of the experiment (especially if the study lasts over a long period of time);
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