Suppose you were operating a machining process that drilled a hole with a specification of .500 plus or minus .003 into a part. That is, the specification is .497 to .503. The quality characteristic that is important is the inside diameter of the hole. Now suppose you measure and record the diameter of every part that is produced. You might find the following sequence of measurements for eight consecutive parts:
.501 .500 .498 .500 .499 .501 .500 .499
As we discussed in Chapter 1, there will be some variation from part to part due to materials, temperature, and so on. We can see this in these measurements. Most are close to the nominal value of .500 and within specifications.