Do you pay for certain Web services? Georgia Institute of Technology's Graphics Visualization and Usability Center surveyed 13,000 Internet users and asked them about their willingness to pay fees for access to Web sites. Of these, 2,938 were definitely not willing to pay such fees (Inc. Technology, No. 3,1995).

a. Assume the 13,000 users were randomly selected.

Construct a 95% confidence interval for the proportlon definitely unwilling to pay fees.

b. What is the w~dth of the interval you constructed in

" part a? For most applicat~ons, this width 1s unneces- x sarlly narrow. What does that suggest about the sur- irl vey's sample size?

C. How large a sample size is necessary to estimate ii the proportion of interest to withln 2% with 95%

"confidence?

Z&

SA2 'Ihe EPA wants to test a randomly selected sample of n water specimens and estimate the mean daily rate of pol lution produced by a mining operation. If the EPA wants a 95% confidence interval estimate with a bound on the error of 1 milligram per liter (mg/L), how many water specimens are required in the sample? Assume prior knowledge indicates that pollution readings in water samples taken during a day are approximately normally distributed with a standard deviation equal to 5 mgIL.