Thomson writes to Ledermann regarding the method of sums of differences, which Thomson uses frequently in his work but with which Ledermann is unfamiliar.

He refers Ledermann to a 1909 paper of Spearman's in which the method was first used, and explains the form of the method he himself has used since, which he refers to as the pooling square. He goes on to discuss Hotelling's paper on the most predictable criterion in the Journal of Educational Psychology.