EMPLOYMENT PSYCHOLOGY 
means or medians of the two distributions, but recognition 
is given only to the compartments into which the measures 
fall and not to their absolute value. The fourfold table 
method is simple but is at best only a rough preliminary indi- 
cation of the amount of correlation. 
Here again we have two formulas. The first is Pearson’s 
cos 7 method, 
(19) = aN T 
Vad ++/bc 
where 7 equals 180°, and a, b, ¢, and d are the numbers of 
cases in each of the four quadrants of the fourfold table. 
a is the number of cases above the central tendency (average or 
median) in both series. 
d is the number of cases below the central tendency in both 
series. 
b is the number of cases above in the first series and below in 
the second series. 
¢ is the number of cases below in the first series and above in 
the second series. 
The second formula for tetrachoric correlation is Shep- 
pard’s method of unlike signs: 
(20) 7z=cos (Ur) 
U is the proportion of paired measurements having unlike 
signs. A pair of measurements has unlike signs if one of the 
pair is above the central tendency in its series and the other 
is below the central tendency in its series. This formula is 
to be used only when the division lines correspond very 
closely to the medians of the two series. 
An example of the use of each of these two formulas is 
given in Figure 21. Table 7 simplifies the computation of 7. 
These two formulas are to be avoided when the correla- 
tion is low and the number of cases is small. They cannot 
be used when the measure of central tendency in one of the 
series falls within a large class interval, making it impossible 
to divide the group into two approximately equal parts. For 
180