Statistical Quality Control On Statistical Design of the S 2 Control Chart

The Shewhart S chart based on three-sigma control limits for monitoring thestandard deviation of the process quality characteristic is commonly used inindustry. The statistical disadvantages of this standard Schart are well known. Forexample, the central line is not an unbiased estimate of the true process . Theuse of three-sigma limits is approximate and often leads to a lower control limit ofzero when the subgroup size (also called sample size) is small, of the order of five.Hence the standard Schart cannot detect any quality improvement or reduction ofprocess variation. Statisticians advocate the use of probability limits in control chartdesign, and recommend the use of the S