Choose the method based on risk, complexity, recurrence, evidence quality, and cross-functional impact, not by habit. A simple 5 Whys may be enough for an isolated issue with clear facts, while 8D, fault tree analysis, or a structured RCCA is more appropriate when the non-conformance is repeated, systemic, supplier-related, or poorly understood. The method must fit your evidence trail, validation burden, and existing QMS workflows.