A national manufacturer of ball bearings is experimenting with two different processes, $A$ and $B$, for producing precision ball bearings. It is important that the diameters be as close as possible to an industry standard.
The output from each process is sampled and the average error from the industry standard is measured in millimeters. The manager is interested in determining whether there is evidence that the two processes yield different average errors. The population standard deviations are unknown but are assumed to equal.
What is the null hypothesis?