EMPLOYMENT PSYCHOLOGY 
tween the groups. The second type of device gives a single 
numerical index of the concomitance of the two variables— 
a coefficient of correlation. It is usually employed when 
both test scores and criterion are continuous variables, 
although some correlation formulas may be used when either 
or both are discrete variables. 
GROUP COMPARISONS 
If the criterion of success is a discrete variable, enabling 
the investigator to divide the subjects into two or three 
groups—the successes, the undistinguished, and the failures 
—the method of group comparisons is indicated. This meth- 
od discloses how well the measuring instrument under inves- 
tigation differentiates these groups. For a measuring instru- 
ment to have prognostic value, the successes and the failures 
must be well differentiated. 
Differentiation of groups by test scores may be studied by 
drawing for each of the groups a distribution curve of scores 
in the test. The distribution curves for the success group 
and the failure group should be drawn on the same chart 
and with the same reference points. The differentiating 
capacity of the test may be observed by noting the over- 
lapping of the two distributions. 
To obtain an accurate index of this differentiation, we 
must first compute the means and standard deviations of the 
distributions of test scores for the vocational successes and 
the vocational failures. The standard deviations of the 
means are obtained by the formula: 
(=\ Hi; Td 
ND 
i= [EE or 
N N 
where # is the deviation of each measure from the mean, d 
any deviation from the mean and f the frequency of that 
deviation, 5, the standard deviation of the distribution, and 
170 
NT