Is that change isolated to the problem location?

You've found a difference in disengagement between the two years at Southfield, and no difference in overtime hours worked. Now you'll check the other locations too, to make sure those locations didn't also have a big change in disengagement. If the disengagement rate increased at the other locations where the accident rate did not change much, you'll have to doubt whether disengagement and accident rates are connected.

To test the other locations all at once, create a new dataset where Southfield is filtered out. Then rerun the same statistical tests from the previous exercises, but use the new dataset to test the other locations.