Overview for Nonnormal Capability Analysis

Use Nonnormal Capability Analysis to evaluate the capability of your process based on a nonnormal distribution. Using this analysis, you can do the following:

  • Determine whether the process is capable of producing output that meets customer requirements.
  • Evaluate the overall capability of the process.

To perform the analysis, you must specify a lower or upper specification limit (or both) to define your process requirements. You must also select a nonnormal distribution to model your data. The analysis evaluates the spread of the process data in relation to the specification limits. When a process is capable, the process spread is small relative to the specification spread. This analysis can also indicate whether your process is on target. In addition, the analysis estimates the proportion of product that does not meet specifications.

For example, administrators at a medical center want to evaluate wait times for patients with scheduled medical appointments. They want to ensure that patients are seen by a physician within 15 minutes of their scheduled appointment time. The administrators fit a Weibull distribution to the process data to analyze its capability.

Where to find this analysis

To perform nonnormal capability analysis, choose Stat > Quality Tools > Capability Analysis > Nonnormal.

When to use an alternate analysis

  • If you do not know which nonnormal distribution best fits your data, use Individual Distribution Identification before you perform this analysis.

  • If you do not know whether your process data are in control or whether they can be evaluated using a specific nonnormal distribution, use Nonnormal Capability Sixpack to evaluate these requirements before you perform this analysis.

  • If want to obtain estimates of within-subgroup variation and potential capability, in addition to overall capability, use Normal Capability Analysis to transform your nonnormal data and analyze process capability based on a normal distribution.