A METHOD OF GRAPHICAL ANALYSIS 



95 



ordinate of 30 on the horizontal movable scale when the zero point 

 of the horizontal movable scale falls at 1000 average message use on 

 the fixed horizontal scale. The point of intersection described in this 

 illustration is indicated on Fig. 9 by P. This characteristic (inter- 



1000 1200 1400 1600 1800 2000 

 AVERAGE MONTHLY MESSAGE USE 



2800 3000 



40 80 120 160 200 240 280 320 360 400 440 480 520 560 



MOVABLE SCALE FOR AMOUNT OF MESSAGE USE (EXPRESSED AS PER CENT OF AM.MU.) SEE NOTE 



Fig. 9 — Simultaneous Smoothing 



sections falling on given vertical lines) forms the basis for smoothing 

 the two families of curves simultaneously. A point of intersection 

 may be shifted vertically but cannot be shifted horizontally since it 



must fall somewhere on a definite vertical line. Dashed lines ( ) 



on Fig. 9 indicate the manner in which a few of the points have been 

 shifted in smoothing. 



A family of cumulative curves may appear easier to smooth than 

 the corresponding family of detail curves. On the other hand, the 

 detail curves give, in some respects, a more vivid picture of the out- 

 standing characteristics of the distributions than do the cumulative, 

 and certain important characteristics of the distributions may be more 

 easily studied on a detail basis. 



It is important, therefore, that both series be taken into account 

 in deriving final normal distribution curves. For the detail series, 

 charts are plotted showing the relationship between the amount of 



