Process Window Index (PWI) is a quantitative metric that measures how centered the current operating point is within the process window — expressed as a percentage where 0% is at the center (maximum margin) and 100% is at the edge (on specification limits).

How PWI Is Calculated

• Per Response: $PWI_i = |(y_i - target_i) / (USL_i - target_i)| imes 100\%$ (for upper half).
• Overall PWI: $PWI = max_i(PWI_i)$ — the worst-case among all responses.
• Interpretation: PWI < 50% = well centered. PWI < 100% = within spec. PWI > 100% = out of spec.
• Composite: Composite PWI combines all responses into a single operating position metric.

Why It Matters

• Process Centering: PWI immediately shows if the process is centered or drifting toward spec limits.
• Monitoring: Track PWI over time to detect drift before reaching specification limits.
• Comparison: Compare PWI across tools or chambers to identify which needs attention first.

PWI is the speedometer for process centering — a single number showing how far the operating point is from the safe center of the process window.