Airline Water Quality Is the water on your airline flight safe to drink? It isn’t feasible to analyze the water on every flight, so sampling is necessary. In August and September 2004, the...

Airline Water Quality Is the water on your airline flight safe to drink? It isn’t feasible to analyze the water on every flight, so sampling is necessary. In August and September 2004, the Environmental Protection Agency (EPA) found bacterial contamination in water samples from the lavatories and galley water taps on 20 of 158 randomly selected U.S. flights (12.7 percent of the flights). Alarmed by the data, the EPA ordered sanitation improvements and then tested water samples again in November and December 2004. In the second sample, bacterial contamination was found in 29 of 169 randomly sampled flights (17.2 percent of the flights).

Is the problem getting worse instead of better? From these samples, we can construct confidence intervals for the true proportion of flights with contaminated water. We begin with the 95 percent confidence interval for π based on the August/September water sample:

Next we determine the 95 percent confidence interval for π based on the November/December water sample:

Although the sample percentage (a point estimate of π) did rise, the margin of error is a little over 5 percent in each sample. Since the confidence intervals overlap, we cannot rule out the possibility that there has been no change in water contamination on airline flights; that is, the difference could be due to sampling variation. Nonetheless, the EPA is taking further steps to encourage airlines to improve water quality.