chpater 13-1 - Chapter 13 Experiments and Observational...

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Chapter 13 Experiments and Observational Studies
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Biased Surveys Bias is any systematic failure of a sample to represent its population.
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Sources of Bias Voluntary Response Non-Response Under coverage
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Voluntary Response In general, internet polls ALL suffer from voluntary response bias There is NO WAY to overcome this bias So, basically, we can NEVER use the results of an internet poll to make predications about the population Because people who voluntarily respond are a special group of people who are passionate about the topic and do not represent the general public
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The Literary Digest Poll The magazine Literary Digest sent a survey to 10 million Americans to determine how they would vote in the upcoming (1936) presidential election between Democrat Franklin Roosevelt and Republican Alf Landon. More than two million Americans responded to this poll, and 60% supported Landon. The magazine published these findings, suggesting that Landon was guaranteed to win the election.
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Who was Alf Landon? Roosevelt defeated Landon in one of the largest landslide presidential elections ever. What happened? Under coverage - poor sampling frame
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Sampling Frame You need to take a sample of the correct population In the Literary Digest poll, the people who were sampled were readers The readers of Literary Digest were, in general, the upper class in the country The sampling frame, therefore, was the upper class and the results told us that the upper class favored Landon
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Designing Surveys Identify the population of interest and design a good sampling plan Write questions that are not likely to produce response bias There is no way to recover from a biased sample -- choose your sample well. But how? Use the good methods of data collection discussed in previous slides.
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Chapter 13 Experiments and Observational Studies
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Chapter 13 Experiments and Observational Studies
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Chapter 13 Experiments and Observational Studies
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This note was uploaded on 03/27/2012 for the course STT 200 taught by Professor Dikong during the Spring '08 term at Michigan State University.

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chpater 13-1 - Chapter 13 Experiments and Observational...

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