7_sampling - Sampling State news online poll Should the...

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Sampling
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State news online poll Should the federal government recognize legal same-sex marriages? 41% Yes 57% No 2% I don't know N=116
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Today: Sampling to learn about populations Some terms How the pros do it Types of Sampling Does it work?
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Sampling is a tool used to learn about a population Instead of doing a complete enumeration, we systematically select a sample to represent the population Used when conducting survey research and many other types of research
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Sample and Population The population proportion is the proportion of people in the population with a given characteristic The sample proportion is the proportion of people in a given sample with a given characteristic We will talk about other sample and population values as well (e.g. means)
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Samples and populations Say we are interested in the proportion of MSU students who oppose gay marriage We conduct a poll in which we sample 100 MSU students 50% in our sample oppose gay marriage Then our sample proportion is .5 The population proportion is the proportion of MSU students who oppose gay marriage (it could differ from .5)
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Margin of error The margin of error is a range of values within which the population value is likely to fall; it varies by sample size The margin of error for a sample of 1000 is about +/-3% That is, if we find that Obama’s approval rating is 50% in a survey of 1000 people, the percentage of people who approve of Obama is likely to be between 47 to 53
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Margin of error The margin of error Rule of thumb : margin of error is 1/sqrt(n) 1/sqrt(1000)= .03=3% The more people surveyed, the more accurate the survey But it is costly to sample people Going from 500 to 1000 people increases accuracy more than going from 1000 to 1500 More on margins of error later in the semester…
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How are samples selected in polls? Gallup’s method
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7_sampling - Sampling State news online poll Should the...

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