The previous section seemed to suggest that a system of 100% inspection, while expensive, would at least ensure that no non-conforming product was ever delivered to the customer. In fact, that is not true either. If the process produces defective items, then it is very likely that some will be shipped.
Inspection, even 100% inspection, is a subjective and fallable procedure. Two different inspectors may well give different results for the same batch. Even the performance of a single inspector may vary according to external factors, for example:
A simple example which can be used to illustrate the difficulty of maintaining a 100% record in inspection is known as the 'F' test. Click on the link below to open a window containing a short passage of text. Either print a copy, or reading directly from the screen, get your colleagues, family, or friends to count the number of times the letter 'F' appears in the text. Limit the time for the task to 1 minute to add a little external pressure. While some people may get the correct answer, we guarantee that there will be some variation in the answers you receive. Consider this variation in results when thinking about how successful 100% inspection is likely to be.
While it is true that for certain safety critical areas, 100% inspection is desirable as an additional safeguard, it should not be the only means of quality assurance. The real solution is to avoid producing defective items in the first place. Statistical process control and the effective use of control charts can help to achieve this goal.