In his letter, dated 3 Mar 1938 and numbered 13 (38), Thomson thanks Ledermann for his previous letter, and responds to his points regarding the reciprocity of persons and tests, referring to Burt's statement that we can always deduce all we want to know about person-factors from an analysis into test-factors, or vice versa, and need only perform whichever analysis is convenient. Thomson disagrees with this, apart from in the case of doubly centred matrices, and illustrates his point with examples throughout.

Also includes a postcard, dated 5 Mar 1938 and numbered 13a (38), in which Thomson asks Ledermann to work out a formula using M S Bartlett's principle of minimising the other factors not used.