VALIDITY OF THE SURVEY METHOD OF RESEARCH. 15 
in other factors than that under observation occur and are not ade- 
quately eliminated, and laboratory studies on the other hand, in 
which variations in other factors are largely prevented. These varia- 
tions due to factors other than that studied do occur in using the 
survey method, but the amount of data obtained by this method is 
sufficient to permit the elimination of such variations by the opera- 
tion of the law of averages. The fact that there is such unanimity in 
the conclusions of investigators using the survey method in all parts 
of the country is, of itself, evidence of the general validity and great 
utility of this method of research. 
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