A random sample of 18 shearing pins is taken in a study of the Rockwell hardness of the pin head. Measurements on the Rockwell hardness are made for each of the 18 , yielding an average value of 47.50 with a sample standard deviation of 1.9. Assuming the measurements to be normally distributed, construct a 95% confidence interval for the mean Rockwell hardness. Click here to view page 1 of the standard normal distribution table. Click here to view page 2 of the standard normal distribution table. Click here to view page 1 of the table of critical values of the t-distribution. Click here to view page 2 of the table of critical values of the t-distribution. The confidence interval is