Process Analysis - Process (Machine) Capability Analysis - Tolerance Limits
Before the introduction of process capability indices in the early 1980's, the common method for estimating the characteristics of a production process was to estimate and examine the tolerance limits of the process (see, for example, Hald, 1952). The logic of this procedure is as follows. Let us assume that the respective quality characteristic is normally distributed in the population of items produced; we can then estimate the lower and upper interval limits that will ensure with a certain level of confidence (probability) that a certain percent of the population is included in those limits. Put another way, given:
- A specific sample size (n), 2. The process mean, 3. The process standard deviation (Sigma), 4. A confidence level, and 5. The percent of the population that we want to be included in the interval,
we can compute the corresponding tolerance limits that will satisfy all these parameters. The Process Analysis module provides facilities that allow the user to "play around" with these parameters to establish tolerance limits. This module also provides an option to compute parameter-free tolerance limits that are not based on the assumption of normality (Scheffe & Tukey, 1944, p. 217; Wilks, 1946, p. 93; see also Duncan, 1974, or Montgomery, 1985, 1991).
See also, Non-Normal Distributions.