Capability Sixpack (Normal)

Summary

The Capability Sixpack: Normal report provides a complete analysis of process stability using six charts including control charts, subgroup charts, process capability charts (both long-term and short-term), and a normal probability plot (for verifying reasonable normality). The analysis also includes only the traditional capability measures of Cp, Cpk, Pp, Ppk - it does not provide any of the standard Six Sigma performance measures (long-term Z, short-term Z, DPMO).

Answers the questions:
  • What is the capability of the process (both long-term and short-term) at the start of the process improvement project?
  • What is the capability of the process (both long-term and short-term) after improvements have been made?
  • Was the process stable during these assessments?
When to Use Purpose
Start of project Perform a baseline capability analysis to determine process performance 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

Continuous Y (output), at least one specification.

How-To

  1. Verify the measurement system for the Y data is adequate.
  2. Establish a data collection strategy to define how you will sample subgroups over time. Ensure you are using rational subgroups whenever possible.
  3. Collect data for the rational subgroups and enter the data into Minitab. In the Minitab worksheet, you can enter all the data in a single column or you can enter each subgroup into a row. Minitab can also directly import from databases, text files, Microsoft Excel, and so on.
  4. You must provide at lease one specification limit to produce the Capability Sixpack (Normal) report.
  5. You can use the Box-Cox transformation if your data are not reasonably normal.

Guidelines

  • Data are assumed to come from a normal distribution; however, the consequences of having non-normal data are not serious if the data are reasonably normal. Badly skewed data can be easily fixed using a transformation, such as the Box-Cox, which is an option in this report.
  • Capability Analysis (Normal) and Capability Sixpack (Normal) are best used together. While the Capability Analysis (Normal) displays more statistics, the Capability Sixpack (Normal) includes graphs for validating process stability and reasonable normality, which are critical when using the performance measures.
  • If you have discrete numeric data from which you can obtain every equally spaced value, and you have measured at least 10 possible values, your data are evaluated as though they are continuous.
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