Researchers at a National Weather Center in the northeastern United States recorded the number of 90° days each year since records first started in 1875. The numbers form a normal shaped distribution with a mean of µ = 9.6 and a standard deviation of s = 1.9. To see if the data showed any evidence of global warming, they also computed the mean number of 90° days for the most recent n = 4 years and obtained µ = 11.85.
Do the data indicate that the past four years have had significantly more 90° days than would be expected for a random sample from this population? Use a one-tailed test with a = .05.