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 preferable.

In case the two series in a fourfold table are discrete variables
 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)