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Chapter 8 QBA

# Chapter 8 QBA - geographic area into primary units Sampling...

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Chapter 8 QBA Reasons to sample: 1. to contact the whole population would be time consuming 2. cost of studying all the items in a population 3. the physical impossibility of checking all items in the population 4. the destructive nature of some tests 5. the sample results are adequate Simple random sample A sample selected so that each item or person in the population has the same chance of being included Systematic random sample A random starting point is selected and every Kth member of the population is selected Stratified random sampling A population is divided into subgroups, called strata, and a sample is randomly selected from each stratum Cluster sampling A population is divided into clusters using naturally occurring geographic or other boundaries. Then, clusters are randomly selected and a sample is collected by randomly selecting from each cluster. You divide the
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Unformatted text preview: geographic area into primary units. Sampling error The difference between a sample statistic and its corresponding population parameter. If you were to determine the sum of these sampling errors over a large number of samples the result would be very close to zero. This is true because the sample mean is an unbiased estimator of the population mean. Sampling distribution of the sample mean A probability distribution of all possible sample means of a given sample size Central limit theorem If all samples of a particular size are selected from any population, the sampling distribution of the sample mean is approximately a normal distribution. This approximation improves with larger samples. *Most statisticians consider a sample of 30 or more to be large enough for the central limit theorem to be employed...
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