SPC is used as a cost minimizing measure as well as improve productivity and enhance customer satisfaction. It reduces process variations that cause interruptions and failures in the manufacturing flow which result in loss of quality productive time and high scrap costs. These internal failures are avoided by use of statistical process techniques such as the average and range control chart.
The control chart is a graphical representation of samples taken periodically at intervals and could be generated for each characteristic of the process you wish to control. A range chart identifies the amount variation in each sample and the two interact to show whether the process is under control or not. However, as with other processes, use of these charts involves some assumptions. First is that the process design is controllable and secondly there is a normal distribution of samples.