# statistics

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Accuracy of price scanners at Wal-Mart. 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 recent study of the accuracy of checkout scanners at Wal-Mart stores in California was conducted (Tampa Tribune, Nov. 22, 2005). At each of 60 randomly selected Wal-Mart 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 Wal-Mart 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 Wal-Mart spokesperson claims that 99% of California Wal-Mart 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.

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