In 1938 and 1939 he began work, along with
Bernard Babington-Smith known as BBS, on the question of
random number generation, developing both one of the first early mechanical devices to produce random digits, and formulated a series of tests for
statistical randomness in a given set of digits which, with some small modifications, became fairly widely used. He produced one of the second large collections of random digits (100,000 in total, over twice as many as those published by
L. H. C. Tippett in 1927), which was a commonly used tract until the publication of
RAND Corporation's
A Million Random Digits with 100,000 Normal Deviates in 1955 (which was developed with a
roulette wheel-like machine very similar to Kendall's and verified as "random" using his statistical tests). In 1937, he aided the ageing statistician
G. Udny Yule in the revision of his standard statistical textbook,
Introduction to the Theory of Statistics, commonly known for many years as "Yule and Kendall". The two had met by chance in 1935, and were on close terms until Yule's death in 1951 (Yule was
godfather to Kendall's second son). During this period he also began work on the
rank correlation coefficient which currently bears his name (
Kendall's tau), which eventually led to a monograph on
Rank Correlation in 1948. In the late 1930s, he was additionally part of a group of five other statisticians who endeavoured to produce a reference work summarising recent developments in statistical theory, but it was cancelled on account of onset of
World War II. ==War-time efforts==