Question

In order to estimate the average electricity usage per month, a sample of 40 residential customers were selected, and the monthly electricity usage was determined using the customers' meter readings. Assume a population variance of 12,100kWh2. Use Excel to find the 98% confidence interval for the mean electricity usage in kilowatt hours. Round your answers to two decimal places and use ascending order. Electric Usage 765 1139 714 687 1027 1109 749 799 911 631 975 717 1232 806 637 894 856 896 1272 1224 621 606 898 723 817 746 933 595 851 1027 770 685 750 1198 975 678 1050 886 826 1176 583 841 1188 692 733 791 584 1163 593 1234 603 1044 1233 1178 598 904 778 693 590 845 893 1028 975 788 1240 1253 854 1185 1164 741 1058 1053 795 1198 1240 1140 959 938 1008 1035 1085 1100 680 1006 977 1042 1252 943 1165 1014 912 791 612 935 864 953 667 1005 1063 1095 1086 810 1032 970 1099 1229 892 1074 579 754 1007 1116 583 763 1231 966 962 1132 738 1033 697 891 840 725 1031

Answer 1

To find sample mean, Excel formula = "AVERAGE(A2: A125)"

sample mean

population variance

and sample size n = 40

The critical value is , Excel formula : " NORNINV(0.99,0,1) "

To compute 98% confidence interval,

Lower bound = Mean - (critical value* standard deviation/ sqrt(n))

Upper bound = Mean + (critical value* standard deviation/ sqrt(n))

a 98% confidence interval for the mean electricity usage in kilowatt hours