stat 20 chap2

stat 20 chap2 - stat20 Chap 2 Observational study 1 15...

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Unformatted text preview: stat20 Chap 2. Observational study 1 / 15 Observational study ◮ Why do we do observational studies if controlled experiments are so good? ◮ Problems with observational studies 2 / 15 Observational study Subjects assign themselves to the treatment/control groups. The investigator just observes what happens. ◮ Study of lung cancer: smokers vs nonsmoker Study of income: choice of majors Recall: in controlled experiments, investigators decide assignment to treatment/control groups. Again compare a treatment (exposed) group and a control group. What are the treatment/control groups for the studies above? 3 / 15 Association Association : Two variables are called associated, if knowing the value of one variable gives you information on the other Ex 1: weight and height Ex 2: height and smoking 4 / 15 Association vs Causation ◮ Association is NOT causation, because of confounding ◮ Confounding: the treatment and control group differ by some factor (other than the treatment) that influences the response/outcome that is studied ◮ Confounding factors must be associated with both exposure and outcome ◮ What can you say about association?...
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stat 20 chap2 - stat20 Chap 2 Observational study 1 15...

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