A researcher obtains a sample ofn= 16 men who have been diagnosed with high blood pressure. The researcher measures each man's blood pressure, then gives each man a 30 minute training session in relaxation therapy. After the 30 minute session, blood pressure is measured again. For this sample, the average blood pressure dropped byM_D= 18points after therapy, withs²= 100. The researcher would like to use the sample data to estimate the mean effect of the therapy for the entire population. What value should be used for the point estimate of the population mean difference?