8 - experimental design

8 - experimental design - Confounding and Internal Validity...

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Confounding and Internal Validity 18:34 The experimental method has the advantage of allowing a relatively  unambiguous interpretation of results  Manipulates the independent variable to create groups that differ in the levels  of the variables and then compares the groups in terms of their scores on the  dependent variable Confounding variable: A variable that varies along with the independent variable 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 involves eliminating possible confounding that results  in alternative explanations Internal validity: When the results of an experiment can confidently be  attributed to the effect of the independent variable 
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Basic Experiments 18:34 Posttest-Only Design A researcher must obtain two equivalent groups of participants, introduce the  independent variable, and measure the effect of the independent variable on  the dependent variable Selection differences: The people selected to be in the conditions cannot  differ in any systematic way  The researcher must choose two levels of independent variable The effect of the independent variable is measured  o The same measurement procedure is used for both groups, so that  comparison of the two groups is possible  o Any difference between the groups on the dependent variable must be  attributed to the effect of the independent variable  Pretest-Posttest Design A pretest is given before the experimental manipulation is introduced  o Makes it possible to ascertain that the groups were equivalent at the  beginning of the experiment  Advantages and Disadvantages of the Two Designs A pretest enables the researcher to assess whether the groups were in fact  equivalent to begin with 
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This note was uploaded on 11/16/2010 for the course EDHD 460 taught by Professor Staff during the Spring '08 term at Maryland.

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8 - experimental design - Confounding and Internal Validity...

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