Six Sigma Green Belt

Process Capability

The Process Capability is a measurable property of a process to the specification, expressed as a process capability index (e.g., C_pk or C_pm) or as a process performance index (e.g., P_pk or P_pm). The output of this measurement is usually illustrated by a histogram and calculations that predict how many parts will be produced out of specification (OOS).

Two parts of process capability are –

1) Measure the variability of the output of a process,

2) Compare that variability with a proposed specification or product tolerance.

Process capability is a predictable pattern of statistically stable behavior where the chance causes of variation are compared to the engineering specifications.

Process Capability Studies
A process capability study attempts to quantify whether a process can consistently meet the standards set by internal or external customers.

Study Procedure – It includes various steps, as

• Select a process to study which is critical and can be selected using several techniques like a Pareto analysis or a cause-and-effect diagram.
  Verify or define the process parameters. Verification of what the process entails, its boundaries, and gain agreement on the process’s definition. Many of these steps are completed when developing a process map.
• Conduct a measurement systems analysis to ensure that the measurement methods produce sound data.
• Select a process capability analysis method like Cpk, Cp, Ppk and Pp.
• Obtain the data and conduct an analysis.
• Develop an estimate of the process capability. This estimate can be compared to the standards set by internal or external customers.

Identifying Characteristics

Characteristics selected to be part of a process capability study should meet certain requirements

Identifying Specifications/Tolerances
The process specifications or tolerances are determined either by customer requirements, industry standards, or the organization’s engineering department.

Developing Sampling Plans
For new processes, for example for a project proposal, a pilot run may be used to estimate the process capability.

Specification Limits
Specification limits are set by the customer, and result from either customer requirements or industry standards.

Verifying Stability and Normality
If only common causes of variation are present in a process, then the output of the process forms a distribution that is stable over time and is predictable.

Process capability indices
It includes Cp and Cpk, who identify the current state of the process and provide statistical evidence for comparing after-adjustment results to the starting point.

• Cp – It measures the ratio between the specification tolerance (USL-LSL) and process spread.
• Cpk – It measures the absolute distance of the mean to the nearest specification limit.
• Cpm – It is also referred to as the Taguchi index. It is more accurate and reliable than the other indices. It focuses on reducing the variation from a target value (T).

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