An Interval for Capturing a High Percentage of the Process with Confidence

Presenter: Scott Kowalski, Senior Technical Trainer, Minitab, Inc., Sanford, FL, USA

Keywords: Capability Analysis, Statistics, Tolerance Intervals

Industry: Automotive, Healthcare, Manufacturing

Level: Intermediate


Many quality practitioners and Lean Six Sigma belts are familiar with the phrase: “we need a 95/99 interval for our process”. This interval is called a tolerance interval. Statistically, it means that you can be 95% confident that 99% of the population is within the interval. For example, what values of a critical to quality (CTQ) can you be 95% confident that 99% of all products will fall between? This can be especially useful for setting the specification limits of a new process/product within DFSS, but also helpful for quantifying an existing process/product.

Interval estimation is used to provide a range of uncertainty for CTQ measures. There are three types of statistical intervals used in practice: a confidence interval, a prediction interval, and a tolerance interval. This session will explain the difference between the three intervals and then focus on the use of tolerance intervals within the LSS framework. Industry examples will be used to motivate and illustrate the usefulness of tolerance intervals as well as show how tolerance intervals relate to capability analysis.

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January 31, 2019
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