Overall capability for Normal Capability Analysis (2024)

Pp is a measure of the overall capability of the process. Pp is a ratio that compares two values:

• The specification spread (USL – LSL)
• The spread of the process (the 6-σ variation) based on the overall standard deviation

Pp evaluates overall capability based on the variation in the process, but not on its location.

Interpretation

Use Pp to evaluate the overall capability of your process based on the process spread. Overall capability indicates the actual performance of your process that your customer experiences over time.

Because Pp does not consider the location of the process, it indicates the overall capability that your process could achieve if it were centered. Generally, higher Pp values indicate a more capable process. Lower Pp values indicate that your process may need improvement.

Caution

For a complete and accurate analysis, use graphs in combination with other capability indices (such as Ppk) to draw meaningful conclusions from your data.

PPL is a measure of the overall capability of the process. PPL is a ratio that compares two values:

• The distance from the process mean to the lower specification limit (LSL)
• The one-sided spread of the process (the 3-σ variation) based on the overall standard deviation

Because PPL uses information about both the process mean and the process spread, it evaluates both the location and the variation of the process.

Interpretation

Use PPL to evaluate the overall capability of your process relative to its lower specification limit. Overall capability indicates the actual performance of your process that your customer experiences over time.

Generally, higher PPL values indicate that the process is capable at the lower tail of its distribution. Lower PPL values indicate that your process may need improvement.

PPU is a measure of the overall capability of the process based on its upper specification limit. PPU is a ratio that compares two values:

• The distance from the process mean to the upper specification limit (USL)
• The one-sided spread of the process (the 3-σ variation) based on the overall standard deviation

Because PPU considers both the process mean and the process spread, PPU evaluates both the location and the variation of the process.

Interpretation

Use PPU to evaluate the overall capability of your process relative to its upper specification limit. Overall capability indicates the actual performance of your process that your customer experiences over time.

Generally, higher PPU values indicate that the process is capable at the upper tail of its distribution. Lower PPU values indicate that your process may need improvement.

Ppk is a measure of the overall capability of the process and equals the minimum of PPU and PPL. Ppk is a ratio that compares two values:

• The distance from the process mean to the closest specification limit (USL or LSL)
• The one-sided spread of the process (the 3-σ variation) based on its overall variation

Ppk evaluates both the location and the overall variation of the process.

Interpretation

Use Ppk to evaluate the overall capability of your process based on both the process location and the process spread. Overall capability indicates the actual performance of your process that your customer experiences over time.

Generally, higher Ppk values indicate a more capable process. Lower Ppk values indicate that your process may need improvement.

Caution

The Ppk index represents only one side of the process curve and does not measure how the process performs on the other side of the process curve.

For example, the following graphs display two processes that have identical Ppk values. However, one process violates both specification limits, and the other process violates only the upper specification limit.

If your process has nonconforming parts that fall on both sides of the specification limits, consider using other indices, such as Z.bench, to more fully assess process capability.

Interpretation

Use Cpm to evaluate the overall capability of your process relative to both the specification spread and the target. Overall capability indicates the actual performance of your process that your customer experiences over time.

Generally, higher Cpm values indicate that your process is more capable. Lower values indicate that your process may need improvement.

You can compare Cpm to other values to get more information about the capability of your process.

• Compare Cpm with a benchmark value to assess the overall capability of your process. Many industries use a benchmark value of 1.33. If Cpm is lower than your benchmark, consider ways to improve your process, such as reducing its variation or shifting its location.

• Compare Ppk and Cpm. If the process is centered on target, the Ppk and Cpm values are approximately equal.

Note

To display confidence intervals, you must click Options and select Include confidence intervals when you perform the capability analysis. Minitab displays a confidence interval or a confidence bound for Cp, Pp, Cpk, Ppk, Cpm, and Z.bench.

Interpretation

Because samples of data are random, different samples collected from your process are unlikely to yield identical estimates of a capability index. To calculate the actual value of the capability index for your process, you would need to analyze data for all the items that the process produces, which is not feasible. Instead, you can use a confidence interval to determine a range of likely values for the capability index.

At a 95% confidence level, you can be 95% confident that the actual value of the capability index is contained within the confidence interval. That is, if you collect 100 random samples from your process, you can expect approximately 95 of the samples to produce intervals that contain the actual value of the capability index.

The confidence interval helps you to assess the practical significance of your sample estimate. When possible, compare the confidence bounds with a benchmark value that is based on process knowledge or industry standards.

For example, a company uses a minimum benchmark value of 1.33 for Ppk to define a capable process. Using capability analysis, they obtain a Ppk estimate of 1.46, which suggests that the process is capable. To further assess this estimate, they display a 95% lower confidence bound for Ppk. If the 95% lower confidence bound is greater than 1.33, they can be extremely confident that their process is capable, even when taking into account the variability from random sampling that affects the estimate.

Note

To display Z.bench measures, you must click Options and change the default output from capability statistics to benchmark Z's when you perform the capability analysis.

Interpretation

Use Z.LSL (overall) to evaluate the overall sigma capability of your process relative to the lower specification limit. Overall capability indicates the actual performance of your process that your customer experiences over time.

Generally, higher Z.LSL values indicate that the process is capable at the lower tail of the distribution. Lower values indicate that your process may need improvement. When possible, compare Z.LSL (overall) with a benchmark value based on process knowledge or industry standards. If Z.LSL is less than your benchmark, consider ways to improve your process.

Note

To display Z.bench measures, you must click Options and change the default output from capability statistics to benchmark Z's when you perform the capability analysis.

Interpretation

Use Z.USL (overall) to evaluate the overall sigma capability of your process relative to the upper specification limit. Overall capability indicates the actual performance of your process that your customer experiences over time.

Generally, higher Z.USL values indicate that the process is capable at the upper tail of the distribution. Lower values indicate that your process may need improvement. When possible, compare Z.USL (overall) with a benchmark value based on process knowledge or industry standards. If Z.USL is less than your benchmark, consider ways to improve your process.

Note

To display Z.bench measures, you must click Options and change the default output from capability statistics to benchmark Z's when you perform the capability analysis.

Interpretation

Use Z.Bench (overall) to evaluate the overall sigma capability of your process.

Generally, higher Z.bench values indicate a more capable process. Lower Z.bench values indicate that your process may need improvement. When possible, compare Z.bench with a benchmark value based on process knowledge or industry standards. If Z.Bench is lower than your benchmark, consider ways to improve your process.

Compare Z.Bench (within) and Z.Bench (overall). When a process is in statistical control, Z.Bench (within) and Z.Bench (overall) are approximately equal. The difference between the two values represents the improvement in process capability that you could expect if the process were brought into control. Z.bench (overall) is sometimes referred to as Z.Bench Long-Term (LT).