> It's a smart bet that the 100 group has more (total) fingers.
Not without knowing the size of each group.
More fingers per capita, sure.
But what was clearly meant was that IQ 90 woodworkers would be more accident-prone than IQ 100 ones (on the same tasks etc). And while I'm sure nobody has done the exact study of counting carpenters' fingers, it would be extremely surprising if this were not true. Armies, especially conscript armies, have mountains of excellent data on pretty similar things, and it's good data in that they assign people to tasks, and get detailed reports of every subsequent screw-up.