Capability Analysis (Binomial)

Summary

Measures the stability of a single process step (or data-collection point) over time and reports on the long-term capability (DPMO and long-term Z) of that step based on the observed average defective rate. You can use this report in projects in which the measurement system is only capable of recording defectives versus nondefectives as a means of measuring performance.

Answers the questions:
  • What is the capability of a single process step (or data-collection point) at the start of the process-improvement project?
  • What is the capability of a single process step, or data-collection point, after improvements have been made?
  • Was the process stable during these assessments?
When to Use Purpose
Start of project Perform a baseline capability analysis on the process to determine the capability of a single process step, or data-collection point, at the start of the project. A baseline analysis helps you set improvement goals for the project.
Mid-project Perform a confirmation capability analysis after improvements have been implemented to confirm that the process performs as predicted.
End of project Perform a capability analysis after implementing controls to obtain a final assessment of process capability, and also to determine whether the improvement goals of the project were attained.

Data

Discrete numeric Y (number of defectives), number of units sampled per lot.

How-To

  1. Define a defective unit (what it is and is not).
  2. Verify you can accurately assess each unit (that is, verify the measurement system).
  3. Establish a data collection strategy to define how you will sample lots over time.
  4. In Minitab, enter the number of observed defectives from each sample (lot) into one column. A defective is a unit that has one or more defects. In another column, enter the number of units in each sample. If the samples are all the same size, you can specify a constant instead of using a separate column.
  5. You can also use a historical process defective rate as the basis of the analysis.

Guidelines

  • The Process Capability: Binomial report requires that you have clear definitions about what constitutes a defect and what does not, and is used for processes in which the measurement system is capable of recording units as defective or not defective.
  • This analysis measures the capability and evaluates the stability of the process, using a P chart. You can optionally run tests for special causes on the P chart.
  • This analysis provides the basic capability statistics of "PPM Def", which is the long-term DPMO usually reported as a metric in Six Sigma projects.
  • The report also includes a value of Z, which is the long-term Z, rather than the short-term Z typically reported as a metric in Six Sigma projects. To obtain a short-term Z, add the shift (many use 1.5) to the reported Z.
  • While Process Capability: Binomial report does not integrate the performance of multiple process steps into a single measure, both the Capability Rollup Report and Minitab’s Six Sigma Product Report integrate multiple steps (either attribute or continuous) into a single capability measure (including adjusting for process irregularities).
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