Stats
Tolerance limits (April 15, 2005). Category:
Quality control
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
07/08/2008.
