Methods15Chapter6Sampling.pptx - Sampling Case Selection as...

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Sampling Case Selection as a Basis for Inference
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Chapter Outline 1. The Sampling Process 2. Principles of Probability Sampling 3. Steps in Probability Sampling 4. Nonprobability Sampling
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Sampling Process: Initial Steps
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Sampling and Inference
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Sampling Population A complete set of persons, objects, or events that share some characteristic Sample A subset of the population Probability Sampling Based on random selection with an equal or known probability of inclusion Nonprobability Sampling Based on nonrandom selection
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Probability and Random Selection What is probability? The odds or chances that something will occur Probability sampling is based on random selection Each case has a known chance of being selected The chances of selecting one case are independent of the chances of selecting another Selection cannot involve physical mixing and human judgment
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Probability Theory Parameter A numerical description of a population Statistic A numerical description of a sample Statistics are estimates of parameters. Best guess. Good news : Parameters are fixed quantities Bad news : Parameters can’t be observed Good news : Statistics are observable Bad news : Statistics are sample-dependent (vary from sample to sample) Probability Theory gives us a formula for estimating how closely the sample statistics are clustered around the true (parameter) value.
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Probability Theory To provide useful descriptions of the population, the sample must contain essentially the same variations that exist in the population. If all members of a population were identical in all respects there would be no need for careful sampling procedures. However, this is rarely the case. A sample of individuals from a population must contain the same variations that exist in the population to be representative.
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Probability and Sampling Error What is a probability distribution?
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  • Fall '15
  • probability sampling

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