The following observations were made on fracture toughness of a base plate of 18% nickel maraging steel [“Fracture Testing of Weldments,” ASTM Special Publ. No. 381, 1965:

328–356 (in ksi in., given in increasing order)]:

69.5 71.9 72.6 73.1 73.3 73.5 75.5 75.7 75.8 76.1 76.2 76.2 77.0 77.9 78.1 79.6 79.7 79.9 80.1 82.2 83.7 93.7 Calculate a 99% CI for the standard deviation of the fracture toughness distribution. Is this interval valid whatever the nature of the distribution? Explain.