80 



BELL SYSTEM TECHNICAL JOURNAL 



plexity of charts increases rapidly with the inclusion of more variables 

 and makes the analysis and estimating complex and cumbersome. 



Fig. 1 is a typical preliminary trial chart from a study of 

 average telephone message use under message rate service. Each 

 data point represents one class of service in a particular exchange. 

 The independent factors taken into account are: 



1. Major Service Classifications 3 — held constant since this 

 chart is for one class only. 



2. Rank of Service 4 — plotted. 



3. Message Allowance — coded. 



CODE -ALLOWANCE 



+ 40-59 

 a 60-79 

 □ 80-99 

 ° 100 8c OVER 



140 



120 



in 



3 



100 



Id 



CD 

 < 

 CO 

 CO 



W 80 



>3 



60 



o 



S 40 

 < 



UJ 



< 20 



10 



70 



20 30 40 50 60 



AVERAGE RANK OF SERVICE 



Fig. 1 — Preliminary Chart Involving Three Variables 



The trend of the relationships between message allowance, rank of 

 service and average message use is fairly well defined on this chart. 



J Business Main Station, Residence Main Station, P. B. X., etc. 

 4 A statistical index indicating the relative ranking of subscribers in accordance 

 with their demands for service. 



