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B01 CH8b.pdf

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Anna Nagy

Chapter 8Experimental Design In the experimental method all extraneous variables are controlledConfounding and Internal Validity The experimental method provides an unambiguous interpretation of results because the independent variable is manipulated by the researcher to create groups that differ in the levels of the variable which are then compared in terms of their scores of the dependant variableAll other variables are kept constant either through experimental controlor randomizationIf the scores of the groups are different can conclude that it was caused by the independent variable because that was the only difference between the groupsA confounding variable is a variable that varies along with the independent variableconfounding occurs when the effects of the independent variable and the uncontrolled variable are intertwined so you cannot determine which variable caused the observed effectGood experimental design eliminates possible confounding that results in possible alternative explanations because only by eliminating competing alternative explanations can we draw a causal relationship from the independent variableInternal Validitywhen the results of an experiment can confidently be attributed to the independent variable and not any alternate explanationsBasic Experiments The simplest experimental design has two variablesthe independent and dependant variablesThe independent has two levelsthe control group and the experimental groupResearchers make every effort to ensure the only difference between groups is the manipulated variableremember experiments involve control over extraneous variable through keeping such variables constant control group or by randomization There are two types of basic experimentsPosttestonly Design and PretestPosttest DesignPosttestOnly Design diagram pg 151Researcher must1 obtain two equivalent groups of participantsto eliminate and potential selection differences the people selected to be in the conditions cannot differ in any systematic way The groups can be made equivalent by randomly assigning participants 2 introduce the independent variablethe researcher must choose 2 levels of the independent variable such as the experimental which receives a treatmentand the control which does not The researcher could also choose to use two different
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