Overview for Nonparametric Capability Analysis

Use Nonparametric Capability Analysis to evaluate the capability of your process without any assumptions about the distribution of the data. Because parametric assumptions usually provide more accurate estimates with less data, you usually choose a nonparametric analysis when no distribution fits the data and no transformation makes a distribution fit the data. 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. 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. In addition, the analysis estimates the proportion of product that does not meet specifications.

For example, a quality analyst wants to evaluate the capability of a bolt manufacturing process. To satisfy customer requirements, the thread length of the bolts should be within 0.1 mm of the target of 20 mm. The analyst uses a nonparametric capability analysis to evaluate how well the process meets the specifications without any assumptions about the distribution of the data.

Where to find this analysis

To perform nonparametric capability analysis, choose Stat > Quality Tools > Capability Analysis > Nonparametric.

When to use an alternate analysis

Usually, parametric assumptions provide more accurate estimates of capability with less data. To consider how well a parametric distribution fits your data sample, use Automated Capability Analysis. If no distribution fits the data, then the analysis uses the nonparametric method.