The National
Institute for Standards and Technology (NIST) mandates that
for every 100 items scanned through the electronic checkout
scanner at a retail store, no more than 2 should have an inaccurate price. A study of the accuracy of checkout scanners at
Walmart stores in California was conducted. At each of 60
randomly selected Walmart stores, 100 random items were
scanned. The researchers found that 52 of the 60 stores had
more than 2 items that were inaccurately priced.
a. Give an estimate of p, the proportion of Walmart stores
in California that have more than 2 inaccurately priced
items per 100 items scanned.
b. Construct a 95% confidence interval for p.
c. Give a practical interpretation of the interval, part b.
d. Suppose a Walmart spokesperson claims that 99% of
California Walmart stores are in compliance with the
NIST mandate on accuracy of price scanners. Comment
on the believability of this claim.
e. Are the conditions required for a valid large-sample
confidence interval for p satisfied in this application? If
not, comment on the validity of the inference in part d.
f. Determine the number of Walmart stores that must
be sampled in order to estimate the true proportion to
within .05 with 90% confidence using the large-sample
method.