Use Of Control Charts In Factories That Are Approaching Six Sigma
The C chart is the most widely used chart in factories that are approaching six sigma. Since the defect rates are very low, binomial- based control charts would require a very large sample, and hence are impractical to use. For six sigma quality，a defect rate of 3.4 PPM would result in a nP chart with a centerline probability 0.0000034. Such a chart would require a very large sample to determine if the process indeed has gone out of control.
Using C charts with well-defined areas of opportunity, such as defects per shift or defects per 10,000 units, can be effective for monitoring quality control in production. In some factories, the discussion has shifted to the number of possibilities of defects, or the number of opportunities. The electronics industry has defined a new C chart metric, the DPMO (defects per million opportunities) chart. A discussion of DPMO concepts and calculations is given in Chapter 4.
A more realistic way to achieve quality control in factories that approach six sigma is to closely couple the total defect reporting to the continuous quality improvement team. The low defect rate of six sigma manufacturing operation would produce a small number of total defects per day, even in a large factory. For example if we assume that a factory produces 5000 PCBs per day, and each PCB requires 2000 operations, that is a total defects opportunity of 10 million operations per day. For the six sigma defect rate of 3.4 defects per million, the total expected defects is 34. The management of the factory can review these defects individually each day, then decide what corrective action is needed, whether immediate, short, or long term. They can use the tools of TQM to monitor, organize, and rank defects and initiate a corrective action plan to reduce them further.