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Chapter 08

# Chapter 08 - Chapter 8 Producing Data Experiments Essential...

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Essential Statistics Chapter 8 1 Chapter 8 Producing Data: Experiments

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Essential Statistics Chapter 8 2 How Data are Obtained Observational Study Observes individuals and measures variables of interest but does not attempt to influence the responses Describes some group or situation Sample surveys are observational studies Experiment Deliberately imposes some treatment on individuals in order to observe their responses Studies whether the treatment causes change in the response.
Essential Statistics Chapter 8 3 Experiment versus Observational Study Both typically have the goal of detecting a relationship between the explanatory and response variables. Experiment create differences in the explanatory variable and examine any resulting changes in the response variable ( cause-and-effect conclusion) Observational Study observe differences in the explanatory variable and notice any related differences in the response variable ( association between variables)

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Essential Statistics Chapter 8 4 Why Not Always Use an Experiment? Sometimes it is unethical or impossible to assign people to receive a specific treatment. Certain explanatory variables, such as handedness or gender, are inherent traits and cannot be randomly assigned.
Essential Statistics Chapter 8 5 Confounding The problem: in addition to the explanatory variable of interest, there may be other variables (explanatory or lurking ) that make the groups being studied different from each other the impact of these variables cannot be separated from the impact of the explanatory variable on the response

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Essential Statistics Chapter 8 6 Confounding The solution: Experiment : randomize experimental units to receive different treatments (possible confounding variables should “even out” across groups) Observational Study : measure potential confounding variables and determine if they have an impact on the response (may then adjust for these variables in the statistical analysis)
Essential Statistics Chapter 8 7 Question A recent newspaper article concluded that smoking marijuana at least three times a week resulted in lower grades in college. How do you think the researchers came to this conclusion? Do you believe it? Is there a more reasonable conclusion?

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Essential Statistics Chapter 8 8 Case Study The Effect of Hypnosis on the Immune System reported in Science News , Sept. 4, 1993, p. 153
Essential Statistics Chapter 8 9 Case Study The Effect of Hypnosis on the Immune System Objective: To determine if hypnosis strengthens the disease-fighting capacity of immune cells.

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Essential Statistics Chapter 8 10 Case Study 65 college students 33 easily hypnotized 32 not easily hypnotized white blood cell counts measured all students viewed a brief video about the immune system
Essential Statistics Chapter 8 11 Case Study Students randomly assigned to one of three conditions subjects hypnotized, given mental exercise subjects relaxed in sensory deprivation tank control group (no treatment)

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