Chapter13

# Chapter13 - Determining the Size of a Sample Sample...

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Ch 13 22 Sample Accuracy Sample accuracy: refers to how close a random sample’s statistic is to the true population’s value it represents Important points: Sample size is not related to representativeness Sample size is related to accuracy
Ch 13 33 Sample Size and Accuracy Intuition: Which is more accurate: a large probability sample or a small probability sample? The larger a probability sample is, the more accurate it is (less sample error).

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Ch 13 44 A Picture Says 1,000 Words ± Probability sample accuracy (error) can be calculated with a simple formula, and expressed as a ± % number. n 550 - 2000 = 1,450 4% - 2% = ±2%
Ch 13 55 How to Interpret Sample Accuracy From a report… The sample is accurate ± 7% at the 95% level of confidence… From a news article The accuracy of this survey is ± 7%…

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Ch 13 66 How to Interpret Sample Accuracy Interpretation Finding: 60% are aware of our brand So… between 53% (60%-7%) and 67% (60%+7%) of the entire population is aware of our brand
Ch 13 77 Sample Size Axioms To properly understand how to determine sample size, it helps to understand the following axioms…

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Ch 13 88 Sample Size Axioms The only perfectly accurate sample is a census. A probability sample will always have some inaccuracy (sample error). The larger a probability sample is, the more accurate it is (less sample error). Probability sample accuracy (error) can be calculated with a simple formula, and expressed as a +- % number.
Ch 13 99 Sample Size Axioms You can take any finding in the survey, replicate the survey with the same probability sample size, and you will be “very likely” to find the same finding within the +- range of the original finding. In almost all cases, the accuracy (sample error) of a probability sample is independent of the size of the population.

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Ch 13 1010 Sample Size Axioms A probability sample can be a very tiny percentage of the population size and still be very accurate (have little sample error).
Ch 13 1111 Population Size e=±3% Sample Size e=±4% Sample Size 10,000 100,000 1,000,000 100,000,00 0 Sample Size and Population Size Where is N (size of the population) in the sample size determination formula? 1,067 600 1,067 600 1,067 600 1,067 600 In almost all cases, the accuracy (sample error) of a probability sample is independent of the size of the population.

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Ch 13 1212 Sample Size and Population Size Does the size of the population, N, affect sample size or sample error? A probability sample size can be a very tiny percentage of the population size and still be very accurate (have little sample error). Population Size e=±3% Sample Size Sample as % of the Population 10,000 100,000 1,000,000 100,000,000 1,067 1% 1,067 .1% 1,067 .01% 1,067 .0001%
Ch 13 1313 Sample Size Axiom The size of the probability sample depends on the client’s desired accuracy (acceptable sample error) balanced against the cost of data collection for that sample size.

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Ch 13 1414 Putting It All Together MR – What level accuracy do you want?
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## This note was uploaded on 04/14/2009 for the course MKT 232 taught by Professor Staff during the Spring '08 term at Illinois State.

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Chapter13 - Determining the Size of a Sample Sample...

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