The Road to Quality Control

IIProbability and Statistical Inference

In order to understand Statistical Quality Control properly, you need to know about statistical inference and Probability. In the theory of Probability, the probability of an event occurring is based on the hypothesis that states “the results occurring under the same conditions are considered the same”. This is a fundamental law of Science: the belief that you'll always get the same results if you repeat the experiment under precisely the same conditions. However, in the case of products made in factories, it cannot be the same as what you do in the laboratory, where it is easy to set the same conditions. Although we try to control the operating conditions in several ways, such as managing the manufacturing process, doing process analysis, coaching people in standard operating procedures, or by trying other control methods, the products produced frequently don't meet the required standard because of a variety of problems caused by the bad effects of those trials themselves. Even if the manufacturing conditions at one time are consistent with those at another time, they are only “relatively the same”. Therefore, it is difficult to avoid variation in the quality as it's impossible to equalize the characteristics of each one and, moreover, it is hard to do this in an economical way. However, we not only can, but we must, minimize, although not exactly, the amount of variability in the quality of products in a unit or lot, as we explain precisely ...