Constructing a p chart for unpaid invoices. The controller's office of a corporation is concerned that invoices that remain unpaid after 30 days are damaging relations with vendors. To assess the magnitude of the problem, a manager searches payment records for invoices that arrived in the past 10 months. The average number of invoices is 2840 per month, with relatively little month-to-month variation. Of all these invoices, 1028 remained unpaid after 30 days
(a) What is the total number of opportunities for unpaid invoices? What is p‾?
(b) Give the center line and control limits for a p chart on which to plot the future monthly proportions of unpaid invoices.