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lecture13_6slides - Statistics 528 Lecture 13 Sampling...

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Statistics 528 - Lecture 13 1 Statistics 528 - Lecture 13 Prof. Kate Calder 1 Sampling Design The idea of sampling: We want to say something about a population - the entire group of individuals that we want information about. To get at this we take a sample - a part of the population that we actually examine in order to gather information. Statistics 528 - Lecture 13 Prof. Kate Calder 2 Why take a sample? It may be impossible to collect information about every member of the population. – Time – Cost – Convenience The information from a sample is often adequate and easier to obtain. Statistics 528 - Lecture 13 Prof. Kate Calder 3 Example: The Lantern wanted to know how Columbus residents rate the movie “The Lord of the Rings: The Return of the King.” They asked 100 people coming out of the first showing of this movie at the AMC Lennox theatre and noted their ratings. Population: Columbus Residents Sample: 100 people attending first show Variable of interest: Rating of movie Statistics 528 - Lecture 13 Prof. Kate Calder 4 Since we make conclusions about a population based on the information obtained from a sample, it is important that the units in the sample are representative of the entire population . The sampling design , the method chosen to select the sample from the overall population, has important consequences. Poor sampling designs can yield misleading conclusions. Statistics 528 - Lecture 13 Prof. Kate Calder 5 A sampling method is biased if it systematically favors certain outcomes. Some commonly used but biased sampling designs are … Statistics 528 - Lecture 13 Prof. Kate Calder 6 1. Voluntary Response Survey A voluntary response sample consists of people who choose themselves by responding to a general appeal. Voluntary response samples are biased because people with strong opinions, especially negative ones, are most likely to respond.

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Statistics 528 - Lecture 13 2 Statistics 528 - Lecture 13 Prof. Kate Calder 7 Example: The ABC news program Nightline once asked their viewers whether the United Nations should continue to have its headquarters in the United States. In order to have their opinions counted, viewers had to call a 1-900 number and pay a small fee. More than 186,000 callers responded and 67% said “No.” A nationwide poll with a proper sampling design found that less than 28 % of US adults want the UN to move out of the United States. Statistics 528 - Lecture 13 Prof. Kate Calder 8 2. Convenience Sampling Convenience sampling chooses the individuals easiest to reach to be in the sample. Examples:
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lecture13_6slides - Statistics 528 Lecture 13 Sampling...

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