ANATOMY OF A CONFIDENCE INTERVAL FOR A POPULATION PROPORTION (p)

1. ANATOMY OF A CONFIDENCE INTERVAL FOR A POPULATION PROPORTION (p) The (1 - a)100% confidence interval for p, a population proportion, is obtained using the following formula Obtained from the Standard Normal distribution table for the given confidence level (see Anatomy of Obtaining ). Estimate of the proportion being sought (p-hat) Formula components: Example: A survey found that out of 200 workers, 168 said that they were interrupted three of more times an hour by phone messages, faxes, etc. Find the 90% confidence interval of the population proportion of workers who are interrupted three or more times an hour. (Source: information from a USA Today Snapshot, August 6, 1997.) Given: Recall that for za/2 the subscript represents the area in the tails of the standard normal curve and za is the area in the right tail.. So a z-score associated with z.05 = 1.645 Calculation: The Interval: The 90% confidence interval for the proportion of workers who are interrupted 3 or more times per hour may be presented in any of the following ways: 90% C.I. = or (.797, .883) or