964 



THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1953 



ago 



0.70 



0.60 



0.40 



a3o 



0.20 



0.1A 



0.2A 



0.5A 



0.3A 0.4A 0.5A 0.1A 0.2A 0.3A 0.- 



DEVIATION OF x' FROM NOMINAL 



Fig. 8 — Operating characteristic curves for batch method. 



standard deviation to meet its limit, each lot is judged solely on the data 

 obtained from the sample from the lot. The OC curve for the batch 

 method (Criterion III) is shown in Fig. 8(a) for the case where a' = 0.3A. 

 For comparison purposes the corresponding curve for Criterion II of the 

 control chart method is also shown. It is noted that the batch method 

 gives a somewhat sharper discrimination between good and bad distri- 

 butions than Criterion II of the control chart method, due primarily to 

 the use of a relatively larger sample. 



Fig. 8(b) shows how the OC curve is modified for other values of 

 process standard deviation, a'. It is seen that the batch method is rela- 

 tively less sensitive than the control chart method to changes in a pro- 

 vided the deviation from a standard value of d" = 0.3 A is not too great. 



4.3 CHARACTERISTICS OF THE THREE-CELL METHOD 



The operating characteristics of the three-cell method cannot be evalu- 

 ated probability-wise in the manner given for the other two methods. 



