In the Box population experiment this year, the nominal 90% confidence interval using the simple random sample was independently constructed 27 times. Of these intervals, 24 included the true population mean. Let p be the true probability that this interval, based on a SRS and properly constructed, includes the true mean m.

a. Ignoring any concerns you may have with the Normal approximation, compute and interpret the approximate (Wald) 95% confidence interval for p.

b. Repeat problem (a) using the Agresti-Coull interval.

c. Obtain an exact 95% confidence interval for p, using Minitab’s one-sample proportion item under Stat Þ Basic Stat.

Solution: a. Wald’s approximate confidence interval is defined by

where corresponds to the critical value for the standard normal distribution for , is the sample proportion, and is the sample size. Plugging these values in the formula we obtain:

which corresponds to the 95% confidence interval for p. This means that this interval contains the true value of with approximately 95% confidence.

b. Agresti-Coull interval “adds two successes and two failures” to Wald’s interval. It’s defined as

where again, and , which means that

c. Using Minitab we obtain

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