chapter7 - Samplesand observationalstudies PSLS chapter 7...

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    Samples and  observational studies PSLS chapter 7 © 2009 W. H. Freeman and Company
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Objectives (PSLS chapter 7) Samples and observational studies Observation versus experiment Population versus sample Sampling issues Random sampling designs Sample surveys Comparative observational studies
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Observation versus experiment Observational study: Record data on individuals without attempting to influence the responses. Interpretation of results may be confounded with possible lurking variables. 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. 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 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 to yield clear conclusions because the explanatory variable is confounded with lurking variables. studying intelligence Good grade on test CAUSE? Confounding? Well-designed studies take steps to defeat confounding.
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Sometimes the nature of the research problem may require us to use observational study: For example: The effect of mothers’ smoking during pregnancy on academic performance of their children in High School. It is not acceptable to ask a group of pregnant women to smoke while another group is given placebo cigarettes. We therefore need to locate women who smoked of their own will during their pregnancy and compare their children with those of women who did not smoke. Possible confounding factors: If women who choose to smoke during pregnancy tend to have a lower level of education than those of non-smokers, the lower education level may itself affect the children’s academic performance. These two factors would therefore be confounded in an observational study. Comparing subgroups with similar education level would help interpreting the results but other variables could still be confounded.
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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
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chapter7 - Samplesand observationalstudies PSLS chapter 7...

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