Vol. 12, No. 3
March 2010
PQ Systems
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Quality Quiz from Professor Cleary

"B" is correct.

The third conclusion is that the distribution of sample proportions would form a normal distribution. This is the basis for building confidence intervals around a single sample proportion. The diagram below shows 10 sample proportions with confidence limits of plus or minus two standard deviations. If the binomial central limit theorem is true, one would expect about 95 percent of such confidence intervals to include the mean of the population, .50 in this case.

Remember—in a normal distribution, moving plus and minus two standard deviations would capture about 95% of the area under the curve.

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