Company XYZ, a Six Sigma listed company, receives and delivers mails across the United States. Once each day, a sample of 125 mails delivered is drawn from the “Delivery” log that the company maintains and is examined by the Logistics Manager of that company.

The Delivery log consists of all mails that are delivered each day by the company. Those mails that have been delivered to the wrong address or not delivered on-time are classified as defective and the rest are classified as satisfactory. This is one of the ways the manager assesses the quality of their operation. Table 1.11 gives the data on samples of 125 mails collected on 9 different days.

The company wishes to monitor the proportion of defective mail deliveries on a daily basis.

a. Prepare a p chart, with n = 125, for daily monitoring of the defective mail delivery. Use the established control limits to visually represent the chart.

b. After determining the control limits, another sample of 125 mails is taken and is observed to have 95 satisfactory mails. Is corrective action necessary?

c. Is the sample size sufficient for monitoring the defective mail delivery on a daily basis?

Table 1.11:

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Data on Daily Mail Delivery (Exercise 1.9) Sample no. 3 # Satisfactory 109 1 110 2 112 4 114 5 112 6 111 7 105 8 100 9 104