I EMPLOYMENT PSYCHOLOGY 
which are below the assumed mean are prefaced by a negative 
sign. Record in columns 16 and 17. 
10. ZX,,s is the sum of the numbers to the right of the assumed 
mean in row NV. 
11. ZX, is the sum of the numbers to the left of the assumed 
mean in row NV. 
12. Add the above two figures algebraically and record after 
Xx. 
13. Add the numbers in row P on both sides of the assumed 
mean and record this after 2x2. 
14. Perform the operations for the y variable analogous to those 
in 10, 11, 12, and 13, above, and record in the appropriate places 
on the form. 
15. Values of Zxy,,, and Zxy,., are obtained by adding col- 
umns 17 and 16 respectively. Zxy is the algebraic sum of these 
two values. 
16. N is the number of cases and is obtained from either col- 
umn 8 or row L. 
17. The remaining operations are self-explanatory. 
Hull (78) and Dodd (45) have constructed calculating 
machines to aid in computing correlation coefficients. 
With practice it becomes possible to estimate 7 roughly 
from the scatter diagram, which is a graphic presentation 
of the measurements of all persons measured in each of the 
two variables whose relationship is being examined. In a 
scatter diagram the test scores are usually plotted as abscis- 
sae and the criterion as ordinates. 
The product-moment formula may take the form: 
ly 
(13) v= Siz. ye 
With the use of this form it is not necessary to make a scat- 
ter diagram (157, p. 274). 
Correlation may also be computed without scatter dia- 
gram if gross measures or raw scores are used instead of 
deviations from the mean (86, p. 164): 
>XY-(CX)(ZY)/N 
(19) r= SGN VV (2V)N 
=X:—(2X)?/N- VI? —(2Y)*/N