Question 7.40

7.40 How to assemble a new machine.

You ran a two-sample study to compare two sets of instructions on how to assemble a new machine. You randomly assign each employee to one of the instructions and measure the time (in minutes) it takes to assemble. Assume that ${\bar{x}}_{1} = 110, {\bar{x}}_{2} = 120, s_{1} = 8, s_{2} = 12, n_{1} = 20,$ and $n_{2} = 20$ . Find a 95% confidence interval for the average difference in time using the second approximation for degrees of freedom.