Question

In the Federal Trade Commission (FTC) \Price Check" study of electronic checkout scanners, the
FTC inspected 1,669 scanners at retail stores and supermarkets by scanning a sample of items at
each store and determining if the scanned price was accurate. The FTC gives a store a \passing
grade" if 98% or more of the items are priced accurately. Of the 1,669 stores in the study, 1,185
passed inspection.

a. Find a 90% condence interval for the true proportion of retail stores and supermarkets with
electronic scanners that pass the FTC price-check test. Interpret your result.

Answer 1

Use a confidence interval formula for proportions. Here's one example:
CI99 = p + or - (2.58)(sqrt of pq/n)
...where sqrt = square root, p = x/n, q = 1 - p, and n = sample size.

Remember you need a 90% confidence interval; my example uses a 99% confidence interval. You will need to adjust accordingly.

Another hint: x = 1185, n = 1669

I hope this will help.