The manager of a wedding hall holds that the token counter he set up at the door of the dining room is what ended the crush at the serving tables. Which of the following, if true, most strengthens the manager's view?

Switching the supposed cause off and finding the trouble return, then switching it on and finding it go, is about as close to a test as an ordinary hall can come — and it was done twice, with everything else about the hall unchanged. Who mans the counter is a detail of staffing; a record number of bookings would if anything make the absence of a crush harder to credit to the counter; and the number of guests the room seats was the same before the counter existed.