There are three valves on a trumpet. Even Louis Armstrong wove his magic using only the eight possible combinations of fingering. But if Satchmo blew three times, he could have 512 different combinations of valve depression.
The problem is similar in optimal sequencing, and also for surgeons trying to reach a diagnosis using the fewest laboratory tests in the least time and at the least cost. It has always been so, but suddenly has become relevant because of diagnosis-related groups and of the emerging number, complexity, and cost of diagnostic laboratory studies. The problem of sequencing, or precedent order, in its simplest form would involve only two tests (A and B), in which case there are three possible sequencing strategies. We could perform test A on the first day and test B on the second day. The sequence could be reversed, and finally both tests could be performed on the
EISEMAN B. Sequencing. Arch Surg. 1984;119(3):263. doi:10.1001/archsurg.1984.01390150005001