Independence and Opportunities

 “Do I count all defect types as opportunities for defect?” asked the Black Belt while counting the number of opportunities for defect in a document. “You see, the CTQ for one section on this form is that there is a stamp on the line referred to as the execution box. However, I am also finding other defect types. For example, there may be a stamp in this box, by it is not legible. Or, sometimes the stamp is not completely within the box. These defect types could be caused by different things, so should I consider these to be two other defect opportunities?” The Master Black Belt replied “Excellent question. In fact, there is a very important rule to remember. Opportunities for defect must be independent from each other.” 

“Let’s look at your project. In order for the stamp to be illegible, or for it to be considered outside the box, there must be a stamp. This means that these two defect types are dependant on the stamp. So when we count our defects, it means that we will never have all three defects at the same time. This means that in our DPO calculation, the denominator will incorrectly always be bigger than the numerator. Therefore, it is important that opportunities for defect be independent from each other.”

 The Black Belt then asked:  “But what do I do if it is necessary to go further into details and to know the number of defects for the other defect types?” “Well,” replied the Master Black Belt, “in this case, we could count the number of opportunities for defect only for stamps that are present. For example, here, our standard could be that the stamp is in the box, or that the stamp is legible. In this case, however, we would calculate the Sigma metric separately for these opportunities. For example, we would calculate our DPO for the stamp in the execution box, and another DPO for the quality of the present stamps in the execution box.”>>