XXIX XXX] Introduction liii 



Considering only the equation as far as Everitt's Tables extend, we have 

 (r) = - 16079 + -13557r + -01014r 2 + 01585r 3 -f "OOGSlr 4 + 00470r 5 + "00507^ = 0. 



This leads to r = "9365, but the series indicates that the terms are far from 

 converging rapidly. 



The first 12 tetrachoric functions were then used, the last six being found 

 by the table of p n and q n above, and the value of r was found to be '9152. 

 Then 18 functions were used and gave r '9114. 



Lastly 24 tetrachoric functions were used, and the equation below obtained, 

 which led to r = '9105. 



$ (r) = _ -16079 + -13557r + -01014r 2 -I- 'OlSSSr 3 + -00681r< + 00470r 5 

 + -005077* + -00167r 7 + -003957-" + -00055r + -00315r"> 

 + 000107-" + -00255?- 12 - -00009r' s + -00212r" - 'OOOlor 15 

 + 00174r 16 - -00014r" + -00143r' s - 'OOOllr 19 + -00118j 

 - -00057?- 21 + -00096/---' - -00003/- 2 ' + -00079r. 



It will be seen that even with this very large amount of labour we cannot be 

 sure of having reached a final result*. To obviate this the following table was 

 constructed by Everitt, and there is no doubt that the extension of this table to 

 the whole range of correlation would much simplify the discovery of tetrachoric < 

 At present the calculation of high values of r,, for negative correlations is in hand. 



TABLE XXX (pp. 5227) 



Supplementary Tables for determining High Correlations from Tetrachoric 

 Groupings. (P. F. Everitt, Biometrika, Vol. VHI. pp. 385 395.) 



Using the notation of p. 1, 



N 2irVl- 

 iii the case of a tetrachoric table, or 



d 1 



where F=-^=| e~* Aa \ iff- A -'/ 



vl -; 



.(xli). 



* Mr H. E. Soper working out this example draws my attention to the fact that convergence ia closely 

 given by a form : r n =r co (1 + a .c"), where n is the number of terms used and a and c are constants. 

 Hence (r. - r^ ) (r n+2ra - r a ) = (r n+m - r. )\ 



or r - 





 In oar case take >i = 6, m = 6, and we find 



,, g - = 



The valne r 24 is -9105. In this case a= '1567 and c = '7574, but we cannot assert that these would 

 be constants for all tables. If we use r 12 , r^ and r^,, we find r^ = -9102. 



