Between/within (B/W) capability for Normal Capability Analysis for Multiple Variables

Interpretation

Use Cp to evaluate the between/within capability of your process. Between/within capability indicates the capability that your process could achieve if systemic sources of overall process variation, besides the variation within and between the subgroups, were eliminated.

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

Caution

Because the Cp index does not consider the process location, it does not indicate how close the process is to the target region that is defined by the specification limits. For example, the following graphs show two processes with the same Cp value, yet one process falls within the specification limits and the other does not.

Cp = 3.13

Cp = 3.13

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

Interpretation

Use CPL to evaluate the between/within capability of your process relative to the lower specification limit. Between/within capability indicates the capability that your process could achieve if systemic sources of process variation, besides the variation within and between the subgroups, were eliminated.

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

Interpretation

Use CPU to evaluate the between/within capability of your process relative to the upper specification limit. Between/within capability indicates the capability that your process could achieve if systemic sources of process variation, besides the variation within and between the subgroups, were eliminated.

Interpretation

Use Cpk to evaluate the between/within capability, considering both its location and spread. Between/within capability indicates the capability that your process could achieve if systemic sources of overall process variation, besides the variation within and between the subgroups, were eliminated.

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

Caution

The Cpk 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 with identical Cpk values. However, one process violates both specification limits, and the other process only violates the upper specification limit.

Cpk = min {CPL = 4.58, CPU = 0.93} = 0.93

Cpk = CPL = CPU = 0.93

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

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.

Interpretation

Use Z.LSL (between/within) to evaluate the between/within sigma capability of your process relative to the lower specification limit.

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 (between/within) 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.

Interpretation

Use Z.USL (between/within) to evaluate the between/within sigma capability of your process relative to the upper specification limit.

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 (between/within) 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.

Interpretation

Use Z.Bench (between/within) to evaluate the between/within sigma capability of your process.

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