Overview for Tolerance Intervals (Normal Distribution)

Tolerance intervals are a range of values for a specific quality characteristic of a product that likely covers a specified minimum percentage of current or future product output. Use Tolerance Intervals (Normal Distribution) to determine the range within which a specified proportion of the process measurements is expected to fall when your data come from a normal distribution.

You can create two-sided or one-sided intervals depending on what you want to determine, as follows:
  • Use a two-sided interval to determine the interval that contains a certain percentage of the process measurements.
  • Use a one-sided interval to determine the interval such that a certain percentage of the process measurements will not be less than a lower limit, or will not be greater than an upper limit.

For example, a parts manufacturer wants to evaluate the variability of the width of a part. Technicians randomly sample 30 parts and measure the width in millimeters (mm). The manufacturer uses a two-sided tolerance interval to determine the range within which the widths of parts are expected to fall. If this range is wider than the requirements, the process might manufacture excessive waste.

Where to find this analysis

To perform a tolerance interval analysis, choose Stat > Quality Tools > Tolerance Intervals (Normal Distribution).

When to use an alternate analysis

If you want to calculate tolerance intervals when your data do not come from a normal distribution, use nonnormal tolerance intervals.