(A) Tapping, with a blunt needle, and (B} dotting, using McDougalls dotting apparatus.

The coefficient of correlation between A and B was 48 with one group of children.

According to a well-known theory, this result can be accounted for by assuming the presence of specific elements peculiar to tests A and B respectively, and, in addition, common elements operating similarly in each.

There are many other possible explanations, however ; one other in particular, which this paper investigates, is that a number of elements is conceivable which operate in favour of one test and against the other.

A group of such elements is described in this paper as an interference factor ; it has been already foreshadowed by previous writers 2.