Thomson discusses the difficulties of calculating correlations between persons, referring to Burt's article, Correlations Between Persons, from the British Journal of Psychology, Jul 1937; his own paper, On Complete Families, from the British Journal of Psychology, Jul 1935, in which he states he was the first person to suggest the idea of correlations between persons, as well as the many difficulties in the way of this; and a letter he received from Dr William Stephenson, which has not survived in the collection.

He refers to the work of Burt, specifically in relation to correlating two people who have been given different tests, illustrating his points with calculations throughout.