If n = 280 and ˆ p (p-hat) = 0.35, construct a 99% confidence interval.

Answer 1

Confidence interval = phat +/- z*critical for 99% CI * sqrt (phat * (1-phat)/n)

The confidence interval covers the middle 99% of the area under the normal curve, leaving 0.5% of the normal curve open at each end, so the CI covers the area from 0.5% to 99.5% and the z-critical value would be that which corresponds to the 99.5% probability: 2.58.

Plug the z-critical and given phat and n into the CI equation above to get your 99% CI.