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EXPERIMENTAL DESIGN 1

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Describe the relationship between a confounding variable and the internal validity of an experiment Describe the posttest-only design and the pretest-posttest design, including the advantages and disadvantages of each design 2

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Contrast and compare independent groups design with a repeated measures design Summarize the advantages and disadvantages of using a repeated measures design Describe a matched pairs design, including reasons to use this design 3

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Confounding Variable Another variable that occurs along with the independent variable Is an uncontrolled variable Cannot determine which variable is responsible for the effect 4

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Internal Validity An experiment must be designed and conducted so that only the independent variable can be cause of the results Internal validity exists w hen the results of an experiment can be confidently attributed to the effect of the independent variable 5

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Posttest-Only Design Must: Obtain two equivalent groups of participants Care must be taken to eliminate selection differences Introduce the independent variable Measure the effect of the independent variable on the dependent variable 6

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Pretest-Posttest Design A pretest is given to each group prior to introduction of the experimental manipulation Assures that groups are equivalent at the beginning of the experiment Can quickly measure changes that occur from the pretest to the posttest 8

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Advantages of the pretest-posttest design Mortality (dropout factor) Assess equivalency of groups with small sample size Can be used to select participants for the experiment 9

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Disadvantages of the pretest-posttest design Time consuming and awkward to administer Sensitizes participants to what is being studied - Demand characteristics - Reduces external validity 10

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Independent Groups Design Participants participate in only one group Repeated Measures Design Participants are in all conditions 12

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Advantages and Disadvantages of Repeated Measures Design Advantages Fewer participants Extremely sensitive to statistical differences Conditions are identical because person is own control group Disadvantages Order effect Practice effect Fatigue effect Carryover effect 13

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Counterbalancing Complete counterbalancing Latin squares Time Interval Between Treatments Choosing Between Independent Groups and Repeated Measures Design 14

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Goal is to Match People on a Participant Characteristic Matched to either the dependent measure or a variable that is strongly related to the dependent variable 17

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