EMPLOYMENT PSYCHOLOGY 
number of units sold, the investigator must bear in mind 
contributory variables, such as difficulty of the territory 
assigned to the salesman, possible prejudice against him in 
that part of the country, market for the product in his ter- 
ritory, extent to which that market has been covered, 
amount and character of competition, time the salesman 
has been on the job, and his ability to sustain his sales 
record. Some executives have theories as to the type of 
person who ought to be able to sell for their company (such 
as tall men, or blonds, or college men) and use special in- 
centives with salesmen of this type. All these factors tend 
to reduce the reliability of a salesman’s production as a true 
measure of his ability. 
The contributory variables are scarcely less important 
with workers engaged in routine mechanical tasks. As the 
raw materials supplied to them vary in quality, it becomes 
harder or easier to do the work rapidly and accurately. 
Output may be limited by speed of machines or flow of 
materials. Interruptions in the flow of work, breakdowns, 
and other contingencies are difficult to allow for. Stand- 
ards of inspection are sometimes relaxed or stiffened. Heat- 
ing, lighting, or ventilation may be radically altered. The 
supervisor may be having trouble at home which makes 
him unreasonable in his demands on the workers. Any of 
these complicating circumstances may initiate a tremendous 
upset in group morale which shows itself in marked varia- 
tions of quantity and quality of output even though the 
financial incentive remains the same. The investigator 
must watch all such possible variables and hold them con- 
stant or make appropriate allowances. 
Foremen have been compared as to the amount which 
each is able to produce in his department, but this is ordi- 
narily difficult because of differences in size and nature of 
work of departments. The average production per man 
has sometimes been used as the measure of the foreman’s 
ability. 
4. Performance in standardized examinations. Often it 
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