BIOLOGY IN HUMAN AFFAIRS 



had, after all, been a reliable measure of mental alertness. 

 The need had been for precise determination of the relation- 

 ship between test scores and occupational success. 



The preferred range varies for salesmen of different kinds 

 of products and also with the territories within which they 

 operate. In many occupations it has been shown that there 

 is no upper limit to the optimal intelligence score; but 

 studies of policemen and of machine operatives, as well 

 as of salesmen and of clerical workers where the task is 

 essentially routine, have shown how necessary it is to 

 keep an eye on the upper as well as the lower critical score, 

 in order to avoid anomalous or ambiguous inferences. 



Statistical Methods in Employment Psychology. In developing 

 aids for selection and placement of workers, the statistical 

 method of group dijferences has proved useful, particularly 

 in evaluating items of personal history information which 

 throw light on the applicant's character, temperament, 

 and emotional adaptability to the work for which he is 

 being considered. The method is simple enough in applica- 

 tion.^ The groups compared are the successful and un- 

 successful workers employed at the occupation in question; 

 or those successfully engaged in the work, compared with a 

 sample of the population at large. The proportion of each 

 group answering an item on the application blank in a 

 certain way, or making a certain critical score on a test, is 

 ascertained. The difference in proportions is then computed, 

 and also the standard error of this difference. If the differ- 

 ence in proportions is more than twice the standard error 

 of the difference, it is considered to be significant for this 

 purpose. Then the individual applicant's performance with 

 reference to the item in question is given a weight corre- 

 sponding to the size of this ratio, in the total score. Or, if 

 the number of questionnaire items or test scores to be 



1 Bingham and Freyd, "Procedures in Employment Psychology," Chaps, xiii 

 and XV, McGraw-Hill Book Company, Inc., New York, 1926. 



[142] 



