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Austin Cole February 16, 2010
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Outline I. Sampling a. Bad Sampling Methods b. Random Sampling II. Experiments III. Applying Sample to a Population IV. Simulations V. Confidence Intervals VI. Discussion
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Sampling Population- entire group of individuals about which we want information Sample- part of population from which information is collected
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Unemployment Monthly unemployment rate based on survey of 60,000 households Define population Define unemployed Final percentage
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"Labor Force"
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Bad Sampling Methods Convenience sample-sample of easiest to reach members of population Bias-systematically favoring a certain outcome Voluntary Response Sample-people choose to respond to a general appeal
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Simple Random Sampling Every individual in population has equal chance to be sampled Table of random digits
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Cautions about Sample Surveys Undercoverage-group of the population is left out when choosing sample Nonresponse-individual chosen doesn’t participate Wording of questions
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Experiments Observational Study Experiment-imposes some treatment on individuals to observe their responses Confounding variables-variable whose effects cannot be distinguished Control group
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Randomized Comparative Experiment Online vs. classroom courses
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Random Sampling Exercise 1.Starting on line x, read 2-digit groups until you have chosen 6 restaurants. 2.Ignore groups not in the range and ignore any repeated labels. Starting at line 105: 07, 19, 14, 17, 13, 15
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Thinking about Experiments Placebo effect Double-blind experiment Prospective studies
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From Sample to Population Statistical inference-using fact of a sample to estimate about whole population Parameter-fixed number that describes population Statistic-number that describes a sample Sampling Distribution-distribution of values taken by the statistic in all possible samples of the same size from the same population
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Simulation
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Assessing simulations Shape Center-mean of sampling distribution (g) Spread-standard deviation of sampling distribution g(1- g) n
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Confidence Intervals Percent of all samples will produce an interval containing the true population parameter 68-95-99.7 Rule Margin of error for 95% confidence interval: ĝ(1- ĝ) n 2
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95% Confidence Interval
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Exercise A Gallup poll asked a random sample of 1785 adults if they attended church or synagogue in the last 7 days. Of the respondents, 750 said yes. Find the 95% confidence interval. ĝ(1- ĝ) n ĝ=.42 =.023 95% Confidence Interval:.376 to.466
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Discussion In real world examples, what are some uses of knowing the spread/standard deviation? Other uses/applications for this information? 9,38,44a (7 th edition) 9,38,44a (7 th edition) Homework Problems:
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