A corporation wishes to provide an estimate of the total revenue from sales of a product during a period of three months. For each of the N=123 district offices, the total revenue is available during the corresponding three-month period of the previous year: ?X =128,200. A simple random sample of 13 district offices is taken from the 123 offices of the corporation. The resulting data is presented in the table below :

Three-month Three-month Three-month Three-month data from data from data from data from previous year, current year, previous year, current year, X; yi X; Office Office 550 610 1200 1440 720 780 1350 1570 1500 1600 10 1750 2210 1020 1030 11 670 980 620 600 12 729 865 980 1050 13 1530 2020 928 977