The principal of a three-year diploma institute reports: 'Of the students who stayed with us to the end of the course, every one says the compulsory evening study hour helped him.' Which of the following is the most that the principal's evidence, taken by itself, establishes?

The principal has spoken to one group only — the finishers — so her evidence reaches to the edge of that group and no further, which is the fourth option. That the hour helps a student to last the course would need the leavers to have been asked and found less well served, and they were not asked. That the leavers were dissatisfied is equally unknown; silence is not testimony. And whether the finishers are most of the institute's students depends on how many left, a figure the principal never gives.