Select the analysis options for Normal Capability Analysis

Stat > Quality Tools > Capability Analysis > Normal > Options

In This Topic

Target
Use tolerance of K x σ for capability statistics
Capability index naming convention
Perform Analysis
Display
Title

Target

If your process has a target, enter the value. If you enter a target value, Minitab calculates Cpm, a capability index that also considers how much the data deviate from the target.

Use tolerance of K x σ for capability statistics

Enter the width of the tolerance in number of standard deviations (σ). By default, the tolerance is 6 standard deviations wide (3 standard deviations on each side of the process mean).

Minitab interprets the K value as the width of a two-sided tolerance. If you want to use a one-sided tolerance, enter a two-sided tolerance value that is twice that of the one-sided tolerance. For example, if you want to use a one-sided tolerance of 3σ, enter 6.

Note

If you want the tolerance value that you enter to be the default setting for every capability analysis that you perform, choose File > Options > Control Charts and Quality Tools > Capability Analysis and enter the default tolerance.

Capability index naming convention

In Capability index naming convention, select the convention that Minitab uses to name capability indices.

Six Sigma: Use Six Sigma names for capability indices.
AIAG/VDA: Use AIAG/VDA names for capability indices. The names depend on the selected process stage and capability calculation method.

When you select AIAG/VDA, you can also specify the stage of the capability assessment and the capability calculation method.

Note

For nonnormal methods, including those used in Automated Capability Analysis, you can select the capability calculation method with either naming convention. Capability calculation methods do not apply to nonparametric analyses.

In Process stage, select a stage appropriate for the purpose of your study. The output uses names for the statistics that relate to the process stage.

Process performance: Assess how well the process meets specifications allowing common-cause sources of variation to affect the output. Use a performance study when you do not have to demonstrate that the process is stable.
Process capability: Assess how well the process meets specifications allowing common-cause sources of variation to affect the output. Use a capability study when you have a requirement to demonstrate that the process is stable before you assess the process.
Machine performance: Assess how well a machine meets specifications while controlling for variation that does not arise from within the machine. Usually, you use this type of study to show that a machine is suitable to use in production.

In Capability calculation method, select one of the following.

Z-score method: Use the Z-score method, also called the Minitab method.
ISO method (geometric): Use the ISO method, also called the geometric method.

This table shows the capability indices for the two naming conventions. With AIAG/VDA names, the names also depend on the process stage, and calculation method.

Six Sigma	AIAG/VDA
Cp	Cw, or Cbw for between/within
Cpk	Cwk, or Cbwk for between/within
Pp, Ppk	Pp.Z, Ppk.Z or Pp.G, Ppk.G
Pp, Ppk	Cp.Z, Cpk.Z or Cp.G, Cpk.G
Pp, Ppk	Pm.Z, Pmk.Z or Pm.G, Pmk.G

Note

.Z identifies the Z-score method, and .G identifies the ISO method (geometric).

Tip

If you want the naming conventions and the calculation methods that you enter to be the default setting for every capability analysis that you perform, choose File > Options > Control Charts and Quality Tools > Capability Analysis and enter your selections.

Perform Analysis

By default, Minitab performs both within-subgroup and overall capability analyses. If you do not want to perform one of the analyses, deselect that checkbox.

Within subgroup analysis: Perform the within-subgroup analysis, which indicates the potential (short-term) capability of your process. This analysis estimates how well your process could perform if the shift and drift between subgroups were eliminated.
Note
If you use the Johnson transformation on your data, within-subgroup analysis cannot be calculated. In that case, Minitab reports only overall capability.
Overall analysis: Perform the overall analysis, which indicates the actual (long-term) capability of your process. This analysis estimates what your customer actually experiences.

Display

Select how you want to display the expected and observed out-of-specification values:

Parts per million: Display the values in parts per million (PPM).
Percents: Display the values as percentages.

Select the measures of capability that you want Minitab to display:

Capability statistics: Calculate and display capability indices, such as Cp and Pp.
Benchmark Z’s (σ level): Calculate and display Z.bench values. The choice to use Z.bench often depends on company or industry practices.

Include confidence intervals

Select to display confidence intervals for the capability indices.

Confidence level

Enter a confidence level between 0 and 100. Usually, a confidence level of 95% works well. A 95% confidence level indicates that if you took 100 random samples from the process, you could expect approximately 95 of the samples to produce confidence intervals that contain the actual value of the capability index for the process (if all the process data could be collected and analyzed).

For a given set of data, a lower confidence level produces a narrower confidence interval, and a higher confidence level produces a wider confidence interval. The width of the interval also tends to decrease with larger sample sizes. Therefore, you may want to use a confidence level other than 95%, depending on your sample size, as follows:

If your sample size is small, a 95% confidence interval may be too wide to be useful. Using a lower confidence level, such as 90%, will produce a narrower interval. However, the likelihood that the interval contains the capability index for the process decreases.
If your sample size is large, you may want to consider using a higher confidence level, such as 99%. With a large sample, a 99% confidence level may still produce a reasonably narrow interval while increasing the likelihood that the interval contains the capability index for the process.

Confidence intervals

Select the type of confidence interval or bound that you want to display:

One-sided: Displays lower confidence bounds for capability indices and upper confidence bounds for PPM or % out-of-specification limits. Use one-sided confidence bounds for the following:
- To be more confident that a capability index is greater than a required value. For example, to be more confident that Cp is greater than 1.33.
- To be more confident that a PPM or % out-of-specification limits is less than a required value. For example, to be more confident that PPM Total out-of-specification is less than 100.
Two-sided: Display a confidence interval that has both a lower and an upper confidence bound.

Note

If you want to change which statistics display by default, choose File > Options > Control Charts and Quality Tools > Capability Analysis and change the default display setting.

Title

You can enter a new title for the process capability report.