356 BELL SYSTEM TECHNICAL JOURNAL 



Since the demerit is an element of badness, this index increases in mag- 

 nitude as the quahty grows worse as shown in the upper chart on Fig. 3. 

 It is preferable to have the rate high when the quality is good and low 

 when the quality is bad. This has been taken care of by using the 

 factor (1 — Index) in the rate equation, 



Rate = 10 (1 - Index), (3) 



where the factor 10 is introduced merely to make a convenient scale. 

 This gives a rate of + 10 for a product of perfect quality (i.e. no defects 

 found in the material inspected), a rate of zero when current quality 

 is the same as the average for the base period, and a negative rate when 

 current quality is poorer than that of the base period. This equation, 

 as portrayed by the lower chart of Fig. 3, is merely a numerical way of 

 saying "better than" or "poorer than" base period quality and it also 

 tells how much better or poorer. 



The choice of base period rests on judgment and knowledge of con- 

 ditions and must be made with the eyes open. To take care of evolu- 

 tionary changes in manufacturing conditions for telephone products it 

 has been found desirable to use a moving base period ^ of not longer 

 than five years. The use of a somewhat extended period where possible 

 has the advantage of stability in that it tends to smooth out the high 

 and low spots resulting from temporarily abnormal conditions of 

 production such as are liable to recur in the future. The magnitude 

 of the base period demerits per unit thus establishes a reference level 

 for quality under average conditions.^ 



Quality-Control Feature of the Rate 



Rates obtained from week to week or from month to month are used 

 to indicate whether quality has been controlled. If manufacturing 

 conditions are steady and everything is running smoothly, some definite 

 value of rate can be expected. But even with a perfectly controlled 

 process, there will be fluctuations above and below the expected rate 

 value, fluctuations resulting from the effects of a large number of causes 

 over which the manufacturer has no control. 



2 By a moving base period of 3 years is meant the three years just preceding the 

 current year. With a moving base period the standard of reference (the denominator 

 of the index) will change slightly at the beginning of each year as one year is dropped 

 and a new one, the preceding, is added to the base. 



* The average of past experience is sometimes a suitable estimate of expected 

 quality but its indiscriminate use for this purpose is to be avoided. For products 

 which are reasonably well controlled this estimate will often serve satisfactorily. 

 Primarily the denominator of the index is a magnitude chosen to represent some 

 standard of reference. The numerical rate obtained at any time reflects quality 

 relative to the standard. It is not essential to the rate that the expectancy feature 

 be stressed in this connection. Expectancy is, however, of importance to the control 

 limits discussed in the subsequent paragraphs. 



