A METHOD OF GRAPHICAL ANALYSIS 



91 



Since the average is of much importance in determining frequency 

 curves, it will usually be found that differences will be reduced if the 

 curves are plotted with each interval of the horizontal scale as a per 

 cent, of the average instead of an actual value. For example: 



MAIN STATION 

 MESSAGE RATE SERVICE 



PRIVATE BRANCH EXCHANGE 

 MESSAGE RATE SERVICE 



20 



in 

 a 

 u 

 o S 

 a 

 o 



(0 



100 g 



in 



80 



40 80 120 180 200 240 260 320 400 800 1200 1600 2000 2400 2600 3200 



MONTHLY MESSAGE USE 



NOTE > M.A. - MESSAGE ALLOWANCE. A.M.M.U. - AVERAGE MONTHLY MESSAGE USE 



Fig. 5— Sample Distributions of Subscribers by Absolute Message Uses 



Fig. 5 illustrates distributions of subscribers by message use 

 plotted in terms of actual values for different classes of message 

 rate service. On each chart the average message use is indicated. 

 It will be noted that, in general, the greater the average message 

 use, the greater the spread of the curves, the less the concentration 

 at the peak interval and the less marked the correspondence of 

 the peak with the average value. On Fig. 6 the frequency curves 



