Use a capability analysis (binomial) report to measure the stability of a single process step (or data collection point) over time. You also use this report to measure the long-term capability, DPMO and long-term Z, of that step based on the observed average defective rate. Include this report in projects where 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
|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.
||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.
Your data must be a discrete numeric Y (number of defectives), number of units sampled per lot.
- 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 the 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).
- Define a defective unit (what it is and is not).
- Verify you can accurately assess each unit (that is, verify the measurement system).
- Establish a data collection strategy to define how you will sample lots over time.
- 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.
- You can also use a historical process defective rate as the basis of the analysis.
For more information, go to Insert an analysis capture tool.