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Controlling Extraneous Variables Why should we control experiments? o Variability o We expect that people will be different from one another, which is called error variance o When one group varies from another based on extraneous variables, it can lead to not finding differences where real differences exist or to finding differences, where no real difference exists based on your IV Good and Bad Variability Individual Differences or Error Variance (want them to be small so that we can notice the group differences) ---------------------------------------------------------------------------------- Group Differences (want them to not equal; want them to be big) Experimental Controls make error variance smaller and eliminate differences between the groups caused by EVs Three types of extraneous variables 1. Participant variables: intelligence, personality, gender 2. Experimenter variables: variables of people conducting the experiment
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3. Situational variables: usually occur within experiment or unnoticed external feature; environment Two main methods of controlling EVs 1. Balance the EV(s) evenly between your groups/conditions 2. Keep the EV(s) the same for everyone included in the study Randomization o Random Selection: Ensures that the sample in experiment likely represents the population on important characteristics (Not a control technique) o Random Assignment: The only control technique for unknown EVs Allows all possible participant EVs to be evenly distributed to all groups Method 1 of controlling EVs Methods of Randomization o Flipping a coin o Drawing from a hat o Random Number Table (Appendix D in book, p. 502) o Any method that would allow any possible outcome to occur with equal probability to
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