82 



BELL SYSTEM TECHNICAL JOURNAL 



same time. In constructing the curves, the significance to be attached 

 to any data point depends chiefly on: 



1. The number of individual cases on which it is based. 



2. The probable degree of accuracy of the data. 



CODE 

 DATA POINT MEDIAN ALLOWANCE 

 + 40-59 



60-79 

 ° ■ 80-99 



100 & OVER 



10 20 30 40 50 



AVERAGE RANK OF SERVICE 



Fig. 2 — Preliminary Chart With Averages and Curves 



60 



70 



Since these characteristics are considered simultaneously it is usually 

 advisable to depend on judgment using the averages as a general guide, 

 rather than to rely on any formal mathematical system. The first set 



