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Anna Nagy (133)
Lecture 7

PSYB01 - Lecture 7 Notes

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Anna Nagy

PSYB01 - Lecture 7 CHAPTER 8 EXPERIMENTAL DESIGN LEARNING OBJECTIVES  Describe the relationship between a confounding variable and the internal validity of an experiment  Explain error variance  Describe the post-test only design and the pretest-posttest design, including the advantages and disadvantages of each design LEARNING OBJECTIVES  Contrast an independent groups design with a repeated measures design  Summarize the advantages and disadvantages of using a repeated measures design  Describe a matched pairs design, and state reasons why one might use this design CONFOUNDING AND INTERNAL VALIDITY  Internal Validity: experiment must be designed and conducted so that only the independent variable can be the cause of the results TWO WAYS TO MANIPULATE INDEPENDENT VARIABLE 1. Vary it quantitatively 2. Vary it qualitatively CONFOUNDING AND INTERNAL VALIDITY  Confounding Variable: confounding occurs when the effects of the independent variable and an uncontrolled variable are intertwined so one cannot determine which is responsible for the effect CONFOUNDING VARIABLES Tom Rogers wanted to test a new "singalong" method to teach math to fourth graders (e.g., "I love to multiply" to the tune of "God Bless America"). He used the singalong method in his first period class. His sixth period students continued solving math problems with the old method. At the end of the term, Mr. Rogers found that the first period class scored significantly lower than the sixth period class on a mathematics achievement test. He concluded that the singalong method was a total failure. 1. Identify the independent variable(s). 2. Identify the dependent variable(s). 3. Identify any confounding variable(s). 4. Propose a method to "unconfound" the experiment. ERROR VARIANCE “statistical variability of scores caused by the influence of variables other than your independent variables (extraneous or subject related variables)” (Bordens & Abbott, 2005)  Rarely possible to hold all variable constant  Subjects differ from one another  Environmental conditions fluctuate  Subjects change over time May cause fluctuation in scores that have nothing to do with your IV (produce error variance) BASIC EXPERIMENTS  Posttest-Only Design o Must:  Obtain two equivalent groups of participants - Selection differences  Introduce the independent variable  Measure the effect of the independent variable on the dependent variable POSTTEST-ONLY DESIGN POST-TEST ONLY—HYPOTHETICAL EXPERIMENT Performance on Dependent Measure (Simulated Air Control Task) Control Group Experimental Group (THC) 25 13 24 19 18 22 29 18 19 23 Mean 23 Mean 19 REDUCING ERROR VARIANCE  Hold extraneous variables constant (treat participants exactly the same except for IV)  Random Assignment  Larger sample?  Match participants?  Carefully select levels of IV  Statistical
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