Question: A Centers for Disease Control and Prevention survey about cell phone use noted that 14.7% of U.S. households are wireless-only, which means that the household members use only cell phones and do not have a landline. Suppose that this percentage is based on a random sample of 775 U.S. households (Source: http://www.cdc.gov/nchs/data/nhsrinhsr0 4.pdf).

a) Construct a 91% confidence interval for the proportion of all U.S. households that are wireless-only.

b) Explain why we need to construct a confidence interval. Why can we not simply say that 14.7% of all U.S. households are wireless-only?