Understanding Process Capability in Six Sigma

What is Process Capability?

Process capability is a measure of how well a process can meet its specification limits relative to its natural variation. Understanding process capability helps organizations:

• Predict process performance
• Identify improvement opportunities
• Reduce defects and waste
• Meet customer requirements

Key Process Capability Metrics

Cp (Process Capability)

Measures the potential capability of a process, comparing the specification width to the process spread:

• Cp > 1.33 indicates a capable process
• Cp < 1 indicates an incapable process
• Higher Cp values are better

Cpk (Process Capability Index)

Measures actual capability, accounting for process centering:

• Considers both spread and location
• Always less than or equal to Cp
• Minimum value of 1.33 typically required

Measuring Process Capability

1. Collect process data
2. Verify statistical control
3. Check for normality
4. Calculate capability indices
5. Interpret results

Improving Process Capability

Strategies for enhancement include:

• Reducing process variation
• Centering the process
• Implementing controls
• Upgrading equipment
• Training operators