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chapter8 - PSLS chapter 8 2009 W H Freeman and Company...

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    Designing experiments PSLS chapter 8 © 2009 W. H. Freeman and Company
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Objectives (PSLS chapter 8) Designing experiments Experimental terminology Comparing treatments Avoiding bias Types of randomized designs Ethics and experimentation
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Terminology The individuals in an experiment are the experimental units. If they are human, we call them subjects. The explanatory variables in an experiment are often called factors . A treatment is any specific experimental condition applied to the subjects. If an experiment has several factors, a treatment is a combination of specific values of each factor. The factor may be the administration of a drug. One group of people may be placed on a diet/exercise program for 6 months (treatment), and their blood pressure (response variable) would be compared with that of people who did not diet or exercise.
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If the experiment involves giving two different doses of a drug, we say that we are testing two levels of the factor. A response to a treatment is statistically significant if it is larger than you would expect by chance (due to random variation among the subjects). We will learn how to determine this later. In a study of sickle cell anemia, 150 patients were given the drug hydroxyurea, and 150 were given a placebo (dummy pill). The researchers counted the episodes of pain in each subject. Identify: •The subjects (patients, all 300) •The factors/treatments (hydroxyurea and placebo) •And the response variable (episodes of pain)
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Comparing treatments Experiments are comparative in nature: We compare the response to a treatment versus to: another treatment no treatment (a control) a placebo or any combination of the above A control is a situation in which no treatment is administered. It serves as a reference mark for an actual treatment (e.g., a group of subjects does not receive any drug or pill of any kind).
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