lii 



Tables for Statisticians and Biometricians [XXIX 



Hence the equation for r: 



- 000,6093 = -008,007?- + -006,072?-- - -003,572?-" 

 - •003.357?- 4 . 

 Whence r=- 081 + 043. 

 Or, the association is not definitely significant. 



Illustration (iii). Find the tetrachoric r for the Table of Bradford Parents : 



Mother's Habits. 



Here a brief experience will show the reader that to proceed by tetrachoric 

 functions will require a very large amount of labour. 



We have 



1(1- a,) = t = 543/1696 = '32017 ; \ (1 - a 2 ) = T „' = 635/1696 = -37441, 

 d/N= 476/1696 = -28066. 



We have accordingly the following series of tetrachoric functions — the first 

 6 from the table, the remaining 18 from the difference formula. 



