Cpk Process Capability Index

Overview

Cpk measures how well a process fits within specification limits, accounting for both variation (spread) and centering. Cpk = min((USL - μ) / 3σ, (μ - LSL) / 3σ). Cpk ≥ 1.33 is typically required; Cpk ≥ 1.67 for critical characteristics. Unlike Cp, Cpk penalizes off-center processes.

When to Use

Trigger conditions:

• Assessing whether a manufacturing process can meet customer specifications
• Comparing capability across processes, machines, or time periods
• Qualifying a process for production readiness

When NOT to use:

• When the process is not in statistical control (stabilize first with SPC)
• For non-normal distributions without transformation

Algorithm