Here are my five favorite myths. The past will accurately predict the future. Aside from that impossibility, what the chart does not show is the step efforts required to bring the yield up from its previous low level, and the continuing efforts necessary to hold the gains and make new ones. If a new problem should occur, the chart may reverse direction, and the yield may drop again. Managers should be aware that past data may help to predict the future, but that no prediction technique is completely accurate.
Measurements have no errors.
This one often comes to light when a customer returns an allegedly defective item. One scenario is that you cannot verify the defect and the manager concludes that the customer must be incompetent. Another scenario is that the defect is verified, and the manager blames the work cell. If both you and your customer make correct measurements, a gremlin called gage repeatability and reproducibility may be at work. Simply put, the gages and the people using them are subject to random variations.