Abacs for Determining the Eqmjjrohahle Tetrachoric r. 33 



XXI. Abac to determine oO"r 



•50-65 -75 '-82 -86 -91 93 



•60-70 -80 784 •SS •SO -92 -94 95 -96 



97 980 985 



Scale of i(l-foi) and Jll + a^) 



991 993 

 990 -992 994 -985 



997 



