Quality control charts show a characteristic of the product or service against?

In all production processes, we need to monitor the extent to which our products meet specifications. In the most general terms, there are two "enemies" of product quality:

• deviations from target specifications
• excessive variability around target specifications

During the earlier stages of developing the production process, designed experiments are often used to optimize these two quality characteristics (see Experimental Design); the methods provided in Quality Control are on-line or in-process quality control procedures to monitor an on-going production process. For detailed descriptions of these charts and extensive annotated examples, see Buffa (1972), Duncan (1974) Grant and Leavenworth (1980), Juran (1962), Juran and Gryna (1970), Montgomery (1985, 1991), Shirland (1993), or Vaughn (1974). Two excellent introductory texts with a "how-to" approach are Hart & Hart (1989) and Pyzdek (1989); two German language texts on this subject are Rinne and Mittag (1995) and Mittag (1993).

B. control limits based on three standard deviations in each direction.