VALIDATION OF MEASURING INSTRUMENTS 181 
Below mean Above mean 
in Test L in Test L 
Above ++ 
mean in 
job pro- + | 
ficiency 
rating 
a pp 
Below 
mean in 
job pro- — 
ficiency 
rating 
\/bc 
r =C0S—— ——1 
Vad +/be 
Vad+Vbc V21X24+V13X12 
r =.43 (from Table 7) 
r=cos (Un) 
bpp 
Us Tr ys = 30 
7=.43 (from Table 7) 
Figure 21: Illustrating the computation of 7 by the fourfold table method. 
diagnostic purposes the method of critical scores is prefer- 
able. 
In case the two series in a fourfold table are discrete vari- 
ables and dichotomous, the formula to be used is 
foo ad —be 
"hy = ——————— CT —— 
(a+b) (c+d)(a+c)(b+a) 
he 
2 n 
13 21 
24 12 
21)