quality-nonconformance

Use 5 Whys for simple single-cause failures, Ishikawa diagrams for multi-factor problems, Fault Tree Analysis for safety-critical events, and 8D for recurring or customer-mandated issues. Verify each cause with data rather than opinion, and never accept 'operator error' as a root cause.

Corrective action addresses an existing non-conformance and prevents its recurrence, while preventive action addresses a potential non-conformance identified through trend analysis or risk assessment. FDA expects both, and closing a CAPA requires evidence of implementation plus effectiveness validation.

Rule 1 (one point beyond 3σ) demands immediate action, while Rules 2-4 (runs, trends, alternating patterns) indicate systematic causes requiring investigation before the process goes out of spec. A process within specification limits can still be out of control.

Use-as-is requires documented engineering justification showing no functional impact, and customer approval in aerospace (AS9100) or notification in automotive. Never use it for safety-critical characteristics without formal risk assessment and regulatory notification where required.

CAPAs typically fail because the root cause analysis stopped at symptoms, such as blaming human error, or the corrective action was weak, such as retraining alone. Reopen the CAPA, restart the investigation, and verify effectiveness with at least 90 days of recurrence-free data.

Follow a staged escalation: issue a SCAR requiring an 8D response, place the supplier on watch with increased inspection, require controlled shipping, qualify an alternate source, and finally remove them from the Approved Supplier List if no improvement occurs.