lecture_3_1 - Producing data Design of experiments IPS...

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    Producing data:  Design of experiments IPS chapter 3.1 © 2006 W.H. Freeman and Company
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Objectives (IPS chapters 3.1 and 3.2) Design of experiments Obtaining data Terminology Comparative experiments Caution about experimentation Randomization Completely randomized designs Block designs Matched pairs designs
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Obtaining data We need to obtain data for the purpose of our study. One thing we can do is to use available data. Government statistical offices are the primary source for demographic, economic, and social data (visit the Fed-Stats site at www.fedstats.gov). We do not draw conclusions from anecdotal evidence. - Bob has a severe cold. His roommate takes a garlic tablet every morning and has not had a cold in two years. Bob’s aunt also has a friend who takes garlic tablets daily and has not had a cold in more than a year. Based on these data, Bob decides to start taking garlic tablets. -Anecdotal evidence may not be representative of any larger group of cases.
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Some questions require data produced specifically to answer them. This leads to designing observational or experimental studies.
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Observational study : Record data on individuals without attempting to influence the responses. It is difficult to prove anything this way. Example: Based on observations you make in nature, you suspect that female crickets choose their mates on the basis of their health. Observe health of male crickets that mated. Experimental study : Deliberately impose a treatment on individuals and record their responses. Influential factors can be controlled. Example: Deliberately infect some males with intestinal parasites and see whether females tend to choose healthy rather than ill males.
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Terminology The individuals in an experiment are the experimental units . If they are human, we call them subjects . In an experiment, we do something to the subject and measure the response. The “something” we do is a called a treatment , or factor . The factor may be a particular diet/exercise program. One group of people may be placed on this diet/exercise program for six 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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