AAOM Handbook

MP.08 Define specification limits Context

All processes exhibit variation. The question is whether the variation is both predictable (stable) and acceptable (capable). If either of the conditions is present, an intervention may be warranted. Variation can come from two sources: common causes and special causes. Common cause variation can be thought of as recurring, a normal part of the process operation and output. They are a predictable part of the process and are the result of the process operating in a consistent manner. This consistent, predictable performance, however, may not be meeting specification. The process may be predictably, consistently performing poorly! Special cause variation occurs when the process fails to perform in a predictable, consistent manner. Statistically special causes account for less than 1% of the variation in the process. A control chart is used to identify common and special cause variation and thus to assess the stability of the process. If the process is stable an assessment about the capability of the process to meet specifications is made. This task is performed using a capability histogram. For this to occur, the distribution of the performance measures (process width and location) is compared to the location of the specifications. Purpose To define the target value and the upper and/or lower limits for each measure Quantity One statement that identifies the target and at least one specification for each measure. Quality Specification limits are based on the performance values that stakeholders (suppliers/customers/owners) care about. A specification must include at least an upper limit or a lower limit but may include an upper limit and a lower limit, and may also include a target mean. Resources This task is completed by the manager responsible for the process. Time The task should be completed prior to measures being developed