ECONOMIC QUALITY CONTROL OF PRODUCT 367 



and something about my a's that makes them mine. True, not all of 

 your a's are alike. Neither are all of my a's alike. Each group of a's 

 varies within a certain range and yet each group is distinguishable from 

 the others. This distinguishable and, as it were, constant variability 

 within limits is the second characteristic of control. 



4. Definition of Control 



For our present purpose a phenomenon will be said to be controlled 

 when, through the use of past experience, we can predict, at least 

 within limits, how the phenomenon will be expected to vary in the 

 future. Here it is understood that prediction within limits means that 

 we can state, at least approximately, the probability that the observed 

 phenomenon will fall within the given limits. 



In this sense the time of the eclipse of the sun is a predictable 

 phenomenon. So also is the distance covered in successive intervals 

 of time by a freely falling body. In fact, the prediction in such cases is 

 extremely precise. It is an entirely different matter, however, to 

 predict the expected length of life of an individual at a given age; the 

 velocity of a molecule at a given instant of time ; the breaking strength 

 of a steel wire of known cross section; or numerous other phenomena 

 of like character. In fact, a prediction of the type illustrated by fore- 

 casting the time of an eclipse of the sun is almost the exception rather 

 than the rule in scientific and industrial work. 



In all forms of prediction an element of chance enters. The specific 

 problem which concerns us at the present moment is the formulation of 

 a scientific basis for prediction, taking into account the element of 

 chance, where, for the purpose of our discussion, any unknown cause of 

 a phenomenon will be termed a chance cause. 



II. Scientific Basis for Control 



1. Three Important Postulates 



What can we say about the future behavior of a phenomenon act- 

 ing under the influence of unknown or chance causes? I doubt that, 

 in general, we can say anything. For example, let me ask: "What will 

 be the price of your favorite stock thirty years from today?" Are you 

 willing to gamble much on your powers of prediction in such a case? 

 Probably not. However, if I ask: "Suppose you were to toss a penny 

 one hundred times, thirty years from today, what proportion of heads 

 would you expect to find?," your willingness to gamble on your powers 

 of prediction would be of an entirely difterent order than in the previous 

 case. 



