948 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1953 



Where it is desirable to exercise some control over one or more charac- 

 teristics in addition to the key characteristic, a procedure is provided 

 which requires that control charts be maintained on the additional 

 characteristics. This procedure does not require a controlled distribution 

 for such characteristics, but it does give a statistical record which shows 

 the dynamic behavior of the process and indicates when remedial action 

 is desirable. 



2.3 COMPATIBILITY OF SPECIFICATION REQUIREMENTS AND MANUFACTUR- 

 ING PROCESSES 



As mentioned above, special effort has been made in the L3 project 

 to provide specification limits and manufacturing processes for individual 

 component elements that are mutually compatible. 



In order to determine realistic limits on the value of a particular 

 quality characteristic, it is necessary to collect a reasonable quantity of 

 data from the proposed process to show what it can do if brought into 

 a state of statistical control. This is an area in which close cooperation 

 between the element designer and the production engineer is necessary. 

 Here it is convenient to define the ''natural tolerance" of a process as 

 the extreme range of variation to be expected among individual units of 

 product made in relatively short periods of time, such as in single batches 

 or production lots; mathematically it is taken to be equal to Qa, where a 

 is the basic standard deviation of the process as estimated from the 

 average spread for a series of samples, each selected from a different 

 segment of production. 



If it is found, for instance, that the natural tolerance of the process 

 (6<t) is wider than the expected or desired specified limits, then a funda- 

 mental change either of the process or of the basic design of the com- 

 ponent or both is called for, if mutual compatibility is to be attained. 

 Of course, one way to avoid a major change would be to widen the speci- 

 fication limits. If the needs of the system, however, demand the closer 

 limits, such a simple solution is not possible, and a manufacturing pro- 

 cess or design change must be made. In many cases examined in connec- 

 tion with the IJ3 repeater, the economics of the situation — balancing 

 the component cost against the saving in equalization gear — justified 

 additional effort to improve designs and manufacturing processes to 

 obtain limits for individual component elements narrower than those 

 which initial processes appeared capable of meeting. 



2.4 THE PROBLEM OF MEASUREMENT 



The precision of measurement may have an important influence on 

 the determination of the process capability. Let us assume that the 



