The head of a school sends, after each monthly test, the thirty pupils whose marks were lowest to a remedial hour for the month that follows. She reports that these pupils' marks in the next test rise by more than the marks of the class as a whole rise, and holds that the remedial hour is what raises them. Her reasoning is most open to criticism on the ground that it

The thirty are chosen for standing at the bottom of one test, and a pupil lands at the bottom of one test partly through the accident of that day — an ill morning, a paper that happened to ask what he had not revised. Take the same pupils a month later and most of them will be nearer their own ordinary level, which is upward, and the class as a whole has no such push behind it. Who conducts the hour is not part of the argument; the conclusion is about the hour and not about every pupil, so nothing is generalised to the class; and 'marks' keeps one meaning throughout.