A Ten-step Approach to Constant Quality Improvement

Introduction Statistical Process Control(SPC) entails developing the ability to achieve neverending improvement of quality. Control here means more than meeting this year's budget. It means what Juran[l] refers to as "the habit of improvement", or what Deming[2] calls the "philosophy of continuing improvement". The various techniques of SPC provide the means of achieving improvement on a continuing basis. Benefits of improved quality include increased productivity, better competitive position, and increased employee involvement. Since the measured quality of a product is always subject to a certain amount of variability due to chance, statistical signals make it possible to separate variation due to common (chance) causes from variation outside of this system. The reasons for these assignable, or special, causes of variation outside of the normal pattern can be discovered and corrected. The emphasis is shifted from dealing with problems after they occur, to detection and prevention. As there are two types of variation, there are two ways of improving the system or process. One way is to reduce variation owing to common causes and the other is to reduce variation due to special causes. The statistics do not detect special cause variation, they allow a trained operator to do this. If not apprehended and corrected, this type of variation may recur. The other way of improving the process is to reduce variation owing to common causes. This requires action on the process itself, and this is management's responsibility. A process is operating in statistical control when the only source of variation is due to common causes. Process capability is the ability of a process to meet customer expectations (specifications). The basis for continuing improvement is to bring the process into statistical control by detecting and eliminating special causes of variation, so that performance is predictable.