in spring 2017, data was collected from a random selection of sta 2023 students. one of the question how many concerts they had attended in the past six months. for the 39 randomly selected upperclassmen, the sample mean was 2.31 and sample standard deviation was 2.32. for the 35 randomly selected underclassmen, the sample mean was 2.29 and the sample standard deviation was 2.64. what is the point estimate of the difference in the population mean difference in number of concerts attended between underclassmen and upperclassmen?