An experiment was conducted to determine whether surface finish has an effect on the endurance limit of steel. There is a theory that polishing increases the average endurance limit (for reverse bending). From a practical point of view, polishing should not have any effect on the standard deviation of the endurance limit, which is known from numerous endurance limit experiments to be 4000 psi. An experiment was performed on 0.4% carbon steel using both unpolished and polished smooth-turned specimens. The data are as follows: Endurance Limit (psi) Polished Unpolished 0.4% Carbon 0.4% Carbon 85,500 82,600 91,900 82,400 89,400 81,700 84,000 79,500 89,900 79,400 78,700 69,800 87,500 79,900 83,100 83,400 Find a 95% confidence interval for the difference between the population means for the two methods, assuming