lii 



Tal>l,s fur Statistician* and lii"iii'tri<-i<nis [XXIX 



tlir .(nation for r: 



- -000,6093 = -008,007 r + -006,072^ - -003.572^ 



- 003,357r. 



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. 



I 



a 



CO 



~l. 



a 



I 



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

 functions will require a very large amount of labour. 



We have 



J (1 - a,) = T O = 543/1696 = "32017 ; i(l -I)-T.'- 635/1690 = "37441, 

 djN = 476/1 696 = '28066. 



We have accordingly the following series of tetrachoric functions the first 

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



