Capability Sixpack (Nonnormal)

Summary

The Capability Sixpack (Nonnormal) report provides a complete analysis of process stability using six charts including control charts, subgroup charts, and process capability charts (long-term only), and a probability plot for the selected distribution (for verifying a reasonable fit to the data). The analysis also includes only the traditional capability measures of Pp, Ppk; it does not provide any of the standard Six Sigma performance measures (long-term Z, short-term Z, DPMO).

The Capability Sixpack (Nonnormal) report gives you the option of choosing from 13 different distributions or using a Johnson Transformation for cases where none of the distributions provide an adequate fit to the data.

Answers the questions:
  • What is the capability of the process (long-term only) at the start of the process improvement project?
  • What is the capability of the process (long-term only) 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. The 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 them 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. Determine which distribution best fits your data.You can use the Individual Distribution Identification tool in Minitab for this step.
  5. You must provide at least one specification limit to produce the Capability Sixpack(Nonnormal) report.

Guidelines

  • Because the Capability Sixpack (Nonnormal) report does not use the normal distribution as its basis, you do not need to verify normality.
  • You must select an alternative distribution to model your data. It is important that you select an appropriate distribution, as the performance measures are based on probabilities from the assumed distribution. If you select a poor-fitting distribution, you cannot expect to have very accurate results.
  • The Individual Distribution Identification tool in Minitab can help you select a distribution. This tool runs goodness-of-fit tests for all 13 distributions. Low p-values in these tests indicate poor fits, so select one with a higher p-value.
  • If none of the distributions fit very well, look at a histogram of your data. You probably do not have badly skewed data because one of the distributions would have provided a reasonable fit for almost any skewed data. You may have bi-modal data, which no distribution will fit well. This fact is important information about your process and may help you find a possible solution for your project. If you can find the cause of the bi-modality and eliminate it, you will almost always reduce the process variation at the same time.
  • If you have bi-modal data, a suggestion is to use the Capability Analysis (Normal) report for your baseline, and use the "Observed PPM" as the measure of long-term performance instead of the "Expected PPM." Once you have eliminated the cause of your bi-modal data, you can try the normal report for your final performance analysis, if your data are reasonably normal.
  • Capability Analysis (Nonnormal) and Capability Sixpack (Nonnormal) reports are best used together. While the Capability Analysis (Nonnormal) report displays more statistics, the Capability Sixpack (Nonnormal) report includes graphs for validating process stability and the goodness of fit for the selected distribution, 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, you data are often evaluated as though they are continuous.
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