245 MIGRATION AND BUSINESS CYCLES 
Using the same method of weighting, the indices for all the in- 
dustries under consideration—that is, factory employment, bitu- 
minous and anthracite coal mining, railway maintenance, and 
construction, have been combined into an index of seasonal fluctua- 
tions in ‘‘all selected industries” (Curve ‘“‘g” of Fig. D, Chart 54). 
Limitations. 
Before proceeding further, it may be well to summarize the 
limitations of these indices of seasonal variation. In the first place, 
the object in mind has been to obtain evidence of seasonal fluctua- 
tions which may be applied to the years immediately preceding the 
Great War, since the indices of seasonal variation in migration are 
computed chiefly from data for these years. It has been necessary, 
however, to utilize some employment data applying to more recent 
years. Furthermore, these evidences of employment conditions 
have in some instances been fragmentary and indirect. While care 
has been taken to make the indices as representative of the actual 
conditions as possible, and we have no reason to believe that they 
are inaccurate in material respects, yet the existence of a con- 
siderable margin of possible error must be recognized. 
A further source of possible misinterpretation of the significance 
of seasonal fluctuations in employment lies in the inadequacy of 
the available information concerning the extent of dovetailing of 
employment in various industries. When industries are separately 
considered, the aggregate account of seasonal fluctuation may be 
magnified by the failure to take into account that workers may 
shift from one industry to the other when periods of activity do not 
coincide. On the other hand, the consolidation of data for several 
industries may create the impression of a more uniform seasonal 
distribution of employment than actually exists for most workers. 
It is obvious from the data which we have been examining that 
factory employment is declining in midsummer while activity in 
the outdoor industries is increasing; but, unless idle factory workers 
shift readily to outdoor industries, a consolidation of the data for 
all the important industries may convey an exaggerated impression 
of the degree of seasonal regularity in employment. While such an 
index is useful for present purposes, it is not an adequate measure of 
the variation in employment for individual workers or groups of 
workers. 
We have analyzed separately the seasonal fluctuations, first in 
migration, then in employment. We may now turn to a direct 
comparison of the degree of similarity in these seasonal movements.