EMPLOYMENT PSYCHOLOGY 
where a, b, ¢, and d are the numbers of measures in each of 
the four quadrants. This formula is given by Kelley (86, p. 
259) in other symbols as the product-moment correlation 
between two point distributions. 
Formula 21 may be used when there are two answers to a 
test or question, and when the criterion of vocational accom- 
plishment divides the subjects into only two groups: suc- 
cesses and failures. 
Pearson biserial r (86, p. 245) may be obtained when one 
variate is both quantitative and continuous, while the other, 
though actually quantitative, admits of only two subdi- 
visions, or, more technically, is dichotomous. Biserial 7 
comes into use when the test scores are continuous, while the 
criterion is in terms of success and failure, or those leaving 
the firm and those remaining, and so forth. When for the 
purpose of making group comparisons two distribution 
curves are drawn representing scores of each of these dichot- 
omies in a certain test, biserial » may be computed as an 
additional index of the validity of the test. 
The formula for biserial 7 is as follows: 
(22) ye (M,— My) Pq 
a2 
If the continuous series is represented by x and the dichoto- 
mous by 3, then in the above formula 3, is the mean of the 
x scores made by those in the first y category and p the pro- 
portion of cases in this category, J, the mean of the x scores 
made by those in the second y category and gq the proportion 
of cases in this category, ¢ the standard deviation of the 
total x distribution, and z the height of the ordinate at the 
point of truncation of the normal distribution, cutting off p 
proportion of cases. Some values of z are given in Table 1. 
An illustration of the computation of biserial 7 is given in 
Figure 22, 
Ruml (159) shows an application of this formula to the 
process of setting critical scores and to the determination of 
a degree of vocational competence which will correlate most 
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