D4 (Define and Verify Root Cause)

8D discipline that identifies and proves the root cause through data analysis, testing, or experimentation. Uses tools like 5 Whys, Fishbone Diagrams, data analysis, or designed experiments. Key distinction: Must VERIFY suspected root causes, not just theorize. Verification methods include: recreating the defect by introducing the suspected cause, or confirming defect disappears when cause is eliminated. Also identifies Escape Point (where defect should have been caught). Most analytical and time-intensive 8D step.