Abacs for Determining the Equiprobable Tetrachoric r. 33 



XXI. Abac to determine cr r . 



•60-70 -80.84 -88 '90 -92-84 95 -96 97 «80 985 990 -992 994-995 996 997 998« 



Scale of 4(1 + 00 and \(\ + a$. 

 B. 5 



•999 



