96 



BELL SYSTEM TECHNICAL JOURNAL 



the average and the per cent, of cases falling in a particular interval 

 (expressed as per cent, of the average). Fig. 10 is such a chart for 

 the interval 80-90 per cent, of the average message use. 



3 10 



- 8 



• * • •"" . ■" .1 ■ — ■ 



14 



12 



2 10 



uj 8 



CD 



a. 

 o 



CD 6 

 0) 



a. z 



200 400 



600 800 1000 1200 1400 1600 

 AVERAGE MONTHLY MESSAGE USE 



1800 2000 2200 



Fig. 10 — Preliminary Chart of Detail Series 



200 400 600 800 1000 1200 1400 1600 1800 2000 2200 



AVERAGE MONTHLY MESSAGE USE 

 Fig. 11 — Preliminary Family of Detail Curves 



Cross-sections of a family of curves such as those on Fig. 11 

 give a series of detail frequency curves. Further smoothing may be 

 facilitated by a study of these curves. As an aid in this process of 

 smoothing it is desirable to determine the normal location of the 

 peaks and the normal proportion of cases occurring in the peak in- 

 terval, as these are important characteristics of such curves. These 

 normal values may be determined by plotting these factors against 





