THE L3 SYSTEM — QUALITY CONTROL REQUIREMENTS 951 



in the process average. Thus, two Umiting acceptable distributions, 

 Normal in shape, are defined as indicated in Fig. 2, both having a stand- 

 ard deviation, a", equal to the above-mentioned basic standard deviation 

 of the process, one having an average, X", located Y^^g" below nominal 

 and the other having an average J^^c" above nominal. The over-all 

 limits for individual units then become A/" zh SJ-^o-". If we designate the 

 distance from the nominal to either limit by A^ then the permissible 

 variation in the process average is db 0.1^. 



Our discussions here will be confined to the case of characteristics 

 having substantially Normal distributions and having both maximum 

 and minimum specified limits. In the over-all plan provisions have also 

 been made for characteristics having skew distributions or having a 

 single specified limit, maximum or minimum. 



2.6 EXPRESSION OF DISTRIBUTION REQUIREMENTS 



With the decision to use distribution requirements, the question arose 

 as to how such requirements should be expressed. Would it be adequate 

 to write: 



"The distribution of the individual values of characteristic X of the 

 product shall meet the requirements: (1) average, not outside (iV =b a), 

 and (2) standard deviation, not greater than 6."? Should a clause be 

 added saying that characteristic X "shall be statistically controlled?" 



Recognizing that a requirement is not what is written but what it is 

 interpreted to be, it appeared that such an expression or equivalent is 

 not sufficient to explain the intent nor does it provide criteria for de- 

 termining when any segment of production may be considered conform- 

 ing to the intent. What is meant by "the product?" Does the limiting 

 value on the "average" (or the "standard deviation") apply to each 

 production segment of 10 units, each 50 units, each day's production, 

 each six month's production? Can sampling be used? What criteria are 

 to be used for judging whether the distribution of quality of a portion of 

 product is satisfactory? What should be done with the product if it does 

 not meet the criteria? 



As with any requirement on the collective quality of an aggregation 

 of like articles comprising a product, the use of distribution requirements 

 brings in special problems on the clarification of the intent. What is 

 really wanted is a flow of product such as would be obtained in a series 

 of random samples of units from a process whose average is continually 

 maintained within stated limits (iV ± 0.1 A) and whose standard devi- 

 ation does not exceed 0.3A. 



I 



