The Journal of Environmental Engineering (Feb. 1986) reported on a heat transfer model designed to predict winter heat loss in wastewater treatment clarifiers. The analysis involved a comparison of clear-sky solar irradiation for horizontal surfaces at different sites in the Midwest. The day-long solar irradiation levels (in BTU/sq. ft.) at two midwestern locations of different latitudes (St. Joseph, Missouri, and Iowa Great Lakes) were recorded on each of seven clear-sky winter days. The data are given in the table. Find a 95% confidence interval for the mean difference between the day-long clear-sky solar irradiation levels at the two sites. Interpret the results. Date St. Joseph. Mo. Iowa Great Lakes
21-Dec 782 593
6-Jan 965 672
21-Jan 948 750
6-Feb 1,181 988
21-Feb 1,414 1,226
7-Mar 1,633 1 ,462
21-Mar 1,852 1,698