Optical scanners are used in all supermarkets to speed the checkout process. Whenever the scanner fails to read the bar code on the product, the cashier is required to manually punch the code into the register. Obviously, unreadable bar codes slow the checkout process. Statistical process control is used to determine whether the scanner is working properly. Once a day at each checkout counter, a sample of 500 scans is taken, and the number of times the scanner is unable to read the bar code is determined. (The sampling process is performed automatically by the cash register.) The results for one checkout counter for the past 25 days were recorded.

a. Draw the appropriate control chart(s).
b. Does it appear that the process went out of control? If so, identify when this happened and suggest several possible explanations for the cause.