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Standard error and 95% Confidence Interval calculations

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The State of Michigan tested 766 water samples from private wells in Lapeer County, Michigan,
and found that 112 exceeded the EPA guidelines of 50 ppb.

(a) Find the standard error of the estimated proportion.
(b) Find the 95 percent confidence interval width for the true proportion.
(c) Is normality an issue here? Explain.
(d) Try the Very Quick Rule. Does it work well here? Explain.

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Solution Summary

A set of calculations for water quality samples in Michigan. Step by step instructions for calculating standard error, 95 percent confidence interval. Also includes information regarding normality of the sample as well as the very quick method of calculation.

Solution Preview

I will show you all the steps you need to take in order to determine the values.

a) In order to determine the standard error, the following formula is used:

Standard error = square root of [p(1-p)/n]

Where p is the proportion of the sample. In this case, p = 112/766 = 0.1462

Standard error = square ...

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