Tolerance limits (created 2005-04-15).

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Someone asked me about the difference between control limits and tolerance limits.

I have a web page about quality control models that talks a bit about control limits for a control chart. The word "tolerance" is ambiguous and could mean several things. There is a formal tolerance interval which is a confidence interval for percentile limits of a distribution.

In another context, tolerance limit might represent an engineering specification, where values inside the limit represent parts that will work reliably in the machine or product.

Donald Wheeler talks about specification limits and the voice of the customer. That may be a more desirable choice of language. You could also distinguish between internally derived limits (typically from control charts) and externally derived limits (possibly derived from a consensus control chart, possibly derived by a probability model, possibly derived by ad hockery).

Further reading

• Tolerance intervals for a normal distribution.. Prins J, National Institute for Standards and Technology. Accessed on 2005-04-15. www.itl.nist.gov/div898/handbook/prc/section2/prc263.htm
• Understanding Variation: The Key to Managing Chaos. Wheeler DJ (1993) Knoxville, TN: SPC Press Inc. ISBN: 0945320353. [BookFinder4U link]