process capability ratio

**Process capability ratio** is the **spread-based index Cp that compares specification width to process variation width** - it quantifies potential capability if the process were perfectly centered. **What Is Process capability ratio?** - **Definition**: Cp equals specification range divided by six-sigma process spread. - **Interpretation**: Higher Cp indicates narrower process spread relative to tolerance band. - **Assumption**: Cp does not account for mean offset, so centering errors are invisible in this metric. - **Complement**: Cpk adds centering effect and should always be reviewed with Cp. **Why Process capability ratio Matters** - **Spread Benchmark**: Quickly reveals whether variation magnitude is fundamentally compatible with specs. - **Improvement Direction**: Low Cp indicates variance reduction is required before centering actions matter. - **Technology Comparison**: Useful for comparing intrinsic noise across process options. - **Tolerance Planning**: Supports specification and tolerance negotiations with quantified spread data. - **Control Diagnostics**: Cp versus Cpk gap highlights centering versus spread problem balance. **How It Is Used in Practice** - **Stable Data Requirement**: Calculate Cp only after control-chart evidence shows statistical stability. - **Sigma Estimation**: Use appropriate within-process standard deviation method for short-term ratio. - **Combined Review**: Interpret Cp with Cpk and defect-rate estimates before making business decisions. Process capability ratio is **the potential-width lens of SPC capability analysis** - it answers whether process spread can fit the tolerance, but not whether the process is properly centered.