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Chapter08

# The Basic Practice of Statistics (Paper) & Student CD

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Producing data: sampling BPS chapter 8 © 2006 W. H. Freeman and Company

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Objectives (BPS chapter 8) Producing data: sampling Observation versus experiment Population versus sample Sampling methods How to sample badly Simple random samples Other sampling designs Caution about sample surveys Learning about populations from samples (inference)
Observation versus experiment Observational study: Record data on individuals without attempting to influence the responses. We typically cannot 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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Confounding Two variables (explanatory variables or lurking variables) are confounded when their effects on a response variable cannot be distinguished from each other. Observational studies of the effect of one variable on another often fail because the explanatory variable is confounded with lurking variables. studying intelligence Good grade on test CAUSE? Confounding? Well-designed experiments take steps to defeat confounding.
Apply your knowledge Go to page 192 and work on the following problems: 8.1 Cell phones and brain cancer 8.2 Teaching economics 8.3 TV viewing and aggression

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Population versus sample Sample : The part of the population we actually examine and for which we do have data How well the sample represents the population depends on the sample design. A s tatistic is a number describing a characteristic of a s ample. Population: The entire group of individuals in which we are interested but can’t usually assess directly Example: All humans, all working-age people in California, all crickets A p arameter is a number describing a characteristic of the p opulation. Population Sample
Apply your knowledge Go to page 194 and work on the following problems: 8.4 Sampling students 8.5 The American Community Survey 8.6 Customer satisfaction

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Convenience sampling: Just ask whoever is around. Example: “Man on the street” survey (cheap, convenient, often quite opinionated or emotional now very popular with TV “journalism”) Which men, and on which street?
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