I EMPLOYMENT PSYGHOLOGY 
between the mean of a sub-group and the mean of the total 
of which it is a part is given by Pearson (Biom., Vol. V, p. 
182) as 
22 gt 2n n(M—m)? 
4 NS Hol Sy 
where = is the standard deviation of the total and NV the 
number of cases in the total, ¢ the standard deviation of the 
sub-group and # the number of cases in the sub-group, M 
the mean of the total and » the mean of the sub-group (51, 
p- 114). 
Differentiation of groups by questionnaire items may be 
computed in a similar way. A comparison is made of the 
proportions of each of the two groups (vocational successes 
and failures) giving certain answers to certain questions. 
The formula for the standard error of the difference in pro- 
portions is as follows: 
(10) 6; = p7 ot 2 
where p, is the proportion of one group indicating the an- 
swer, g, the proportion of the same group not indicating the 
answer, p. the proportion of the second group indicating the 
particular answer, gq, the proportion of the second group not 
indicating the answer, and #z, and 7. the numbers of indi- 
viduals in each of the two groups. A difference in propor- 
tions may be considered significant if it is two or more times 
as great as its standard error. A brief example of the use of 
this formula in measuring the significance of group differ- 
ences in answering a questionnaire item, is given below. The 
method is more fully explained in reference 57. 
EXAMPLE: 
Question: Can you do good work while people are looking on? 
Total Number Proportion Proportion 
Number Ans. Yes Ans. Yes Not Ans. Yes 
Successful salesmen 40 (nn) 32 .80 (p) .20(q,) 
Unsuccessful salesmen 46 (n,) 23 .50 (p) .50(q.) 
92