As I expected. Churches (and many other places) would have died out years ago without such good people.
An agreed sample. Say 20-25%. If for example you have 8 identical lights that are mounted at very high positions and 2 of them are tested, then the tester should be able to conclude satisfactory or unsatisfactory using this sample. The sampling is not a guarantee that the one of the 8 lights is not dodgy.
Adam