Pre-control and Some Simple Alternatives

Pre-control, also called Stoplight control, is a quality monitoring scheme similar to a control chart. However, the goal of Pre-control is the prevention of nonconforming units. The appeal of Pre-control is due mainly to its ease of implementation, lack of assumptions, and reported successes. However, it has come under much criticism since many of its decision rules appear ad hoc. In addition, there is some confusion regarding different versions of Pre-control suggested in the literature. This article compares the various Pre-control schemes and contrasts them with more traditional control charts such as acceptance control charts and X charts. Based on this analysis the conditions under which Pre-control is appropriate are laid out. Also, alternatives that retain much of the simplicity of Pre-control but have improved operating characteristics are suggested.