Learn by Doing – Hypothesis Tests and Confidence Intervals

Published: March 18th, 2013

Category: Activity 1: Learn By Doing

The purpose of this activity is to give you hands-on practice in following up a test for the population proportion p in which Ho has been rejected with a confidence interval, and getting a sense of how the confidence interval is a natural and informative supplement to the test in these cases.


Recall from a previous activity the results of a study on the safety of airplane drinking water that was conducted by the U.S. Environmental Protection Agency (EPA). A study found that out of a random sample of 316 airplanes tested, 40 had coliform bacteria in the drinking water drawn from restrooms and kitchens. As a benchmark comparison, in 2003 the EPA found that about 3.5% of the U.S. population have coliform bacteria-infected drinking water. The question of interest is whether, based on the results of this study, we can conclude that drinking water on airplanes is more contaminated than drinking water in general. Let p be the proportion of contaminated drinking water on airplanes.

In a previous activity we tested Ho: p = 0.035 vs. Ha: p > 0.035 and found that the data provided extremely strong evidence to reject Ho. We concluded that the proportion of contaminated drinking water in airplanes is larger than the proportion of contaminated drinking water in general (which is 0.035).

Now that we’ve concluded that, all we know about p is that we have very strong evidence that it is higher than 0.035. However, we have no sense of its magnitude. It will make sense to follow up the test by estimating p with a 95% confidence interval.

This document is linked from More about Hypothesis Testing.