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 indication
 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 Sheppard’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 correlation
 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

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