Zhang, Gongxu (1989, ASQC) Beijing University of Science & Technology, Beijing, China
The Shewhart chart can show the indication of abnormality, but it cannot tell us where and what the abnormality is, i.e. it is not diagnostic. The diagnosis with two kinds of quality is a new diagnosis theory. According to the author [2,3], at any operation there are always two kinds of quality as follows: (1) total quality -- just the quality in common sense, (2) specific quality -- the quality specifically resulted from the current operation itself, not including the influence of preceding operation. Thus, correspondingly, there are: (1) two kinds of control charts, i.e. the Shewhart chart for total quality and the cuse-selecting chart for specific quality; (2) two kinds of process capability index, i.e. the total process capability index (just the one in common sense) for the total quality and the specific process capability index for the specific quality. In fact, the total quality and the specific quality reflect respectively in real state and the ideal state of quality of the current operation under given conditions. Comparing these two kinds of quality, we can diagnose the abnormality at once. The key point is how to measure or judge these two kinds of quality. For this purpose, we can use the two kinds of control chart or the two kinds of process capability index stated above. The former is a real-time diagnosis used to discern which operation is abnormal, the latter is a diagnosis over time, it is especially useful to find out which operations are key operations for improving the quality of the finished product of a production line. This paper includes some interesting practical case studies.