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Unformatted text preview: now because we don’t know the value of the population proportion. (And if we did know the value of p then we would not have taken a sample of 1250 adults to try to estimate it). The 95% confidence level tells us that in the long run, this procedure will produce intervals that contain the unknown population proportion p about 95% of the time. f. Bonus #1: What (approximate) sample size would be necessary to have a (conservative 95%) margin of error of 2%? Margin of error = 0.02 = 1/n => 1/0.02 = n so n = (1/0.02)2 = 2,500 Check your answer out against Table 5.1 from the text page 153. g. Bonus #2: How does the margin of error for a sample of size 1000 from a population of 30,000 compare to the margin of error for a sample of size 1000 from a population of 100,000? The two margin of errors would be the same. The population size has no influence on the accuracy of the sample estimates (provided the population is atleast ten times as large as the sample). 26 5.3 and 5.4 Sampling Methods There are good sampling designs and poor ones. Poor: volunteer, self‐selected, convenience samples, often biased in favor of some items over others. Good: involve random selection, giving all items a non‐zero change of b...
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This document was uploaded on 02/25/2014 for the course STATS 250 at University of Michigan.
- Summer '10