958 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1953 



provision is made for an occasional failure to meet the limit for the sam- 

 ple standard deviation. Thus the batch represented by a sample will be 

 considered conforming to the intent of the specified distribution require- 

 ments, and hence acceptable for this feature if the sample meet the f ol- 

 lo^ving^ criterion: 



Criterion III — Batch Acceptability. 



(a) The average, X, meets the A50 limits; and 



(b) The standard deviation, o-, either (1) meets the S50 limit, or 

 (2) fails to meet the S50 limit but at the same time all of the six 

 preceding consecutive standard deviations meet the S50 limit. 



If a batch fails to meet this criterion, the batch must be inspected 100 

 per cent and acceptance is based on the three-cell method. 



Packaging is handled in the same manner as for the control chart 

 method; individual units in a batch accepted by the batch method are 

 to be selected at random from the batch in groups of 5 each and pack- 

 as such for delivery to the user. 



3.3 THREE-CELL METHOD 



Even with the best of conditions things may go wrong from time to 

 time due to any one of a number of causes — changes in raw materials, 

 irregularities in manual operations, faulty performance of processing 

 equipment, etc. As a result, samples taken under either the control 

 chart method or the batch method will sometimes fail to meet the criteria 

 of the methods. In such times of trouble one solution would be to stop 

 production, find the assignable cause, rectify it, and then resume manu- 

 facture. This solution, though perhaps ideal in one sense, may not be 

 practical for several reasons : 



(a) Considerable time may elapse before the assignable cause is found 

 and corrected; 



(b) Manufacturing schedules may be disrupted; and 



(c) No answer is provided to the question of what to do with the un- 

 controlled product already made. 



What is needed, therefore, is a procedure for dealing with the finished 

 units when the process is in trouble distribution- wise, a procedure which 

 will permit shipment of some of the product and at the same time assure 

 that the portions shipped will have a proper distribution. 



To this end a selection procedure referred to as the three-cell method 

 has been provided. Under this method each unit of product is measured 

 for the characteristic in question and the conforming units are classified 



