5 Whys root-cause analysis

The 5 Whys is a question-driven root-cause analysis technique: you ask “Why did this happen?” repeatedly until you reach a systemic cause rather than a symptom. The name comes from a rule of thumb that five iterations typically reach the root cause, but the right number depends on the problem—it may be three or it may be seven.

The technique was developed by Sakichi Toyoda and is central to the Toyota Production System. It is a core tool in lean, Six Sigma, and ISO 9001 corrective-action processes.

How to run a 5 Whys

1. Write a precise, factual problem statement (avoid vague language).
2. Ask “Why did this happen?” and write the immediate cause.
3. Ask “Why did that happen?” about the cause you just wrote.
4. Repeat until you reach a root cause you can act on—a broken or missing system, process, or standard.
5. Verify the chain by reading it backwards: “[root cause] leads to [cause 4], which leads to [cause 3]...”

Worked example

Problem: A batch of 2,000 tablets shipped to the customer with no certificate of analysis.

• Why 1: The shipping team released the batch before receiving the CoA. → Because the CoA was not in the batch folder at release time.
• Why 2: The CoA was not in the batch folder at release time. → Because QA had not yet completed the final review.
• Why 3: QA had not yet completed the final review. → Because the batch record was not routed to QA until 4 hours before the delivery deadline.
• Why 4: The batch record was not routed to QA until 4 hours before the deadline. → Because the routing SOP does not specify a minimum QA review window before scheduled ship date.
• Why 5 (root cause): The routing SOP does not specify a minimum QA review window. → No such requirement exists in any documented procedure.

Corrective action: Add a rule to the batch release SOP requiring QA routing at least 24 hours before planned ship. Add a system check that flags batches scheduled to ship without a completed QA review.

Common exam traps

• The 5 Whys does not guarantee the unique root cause. Different teams asking the same questions may reach different branches. Use a fishbone diagram first to map all hypotheses, then use 5 Whys to drill into the most likely branch.
• Stop when you reach something you can change. “Because humans make mistakes” or “Because the equipment is old” are dead ends if no action follows. Keep asking until you reach a systemic gap.
• Jumping to solutions too early. The urge is to fix the symptom at Why 2 or 3. Resist until you reach the root.
• Multiple root causes are common. A complex problem often has more than one causal chain. Map each branch separately.

More: 5 Whys · 8D problem solving · AOQ & AOQL · APQP & PPAP · AQL sampling plans · Attribute agreement & kappa · Attribute control charts · Availability & maintainability · Benchmarking types · Binomial vs Poisson · Blocking & randomization · Calibration & traceability · Capability: non-normal · Chi-square test · Confidence intervals · Control chart selection · Control plan · CA effectiveness · Cost of quality · Cp vs Cpk vs Cpm · Cpk vs Ppk · CQE exam day tips · CQE study plan · CQE vs CQA vs CSSBB · Is the CQE worth it? · CUSUM & EWMA · Design reviews · Design V&V · Destructive testing · DFMEA vs PFMEA · Document control · Fault tree analysis · First article inspection · Fishbone diagram · FMEA: RPN vs AP · Full vs fractional factorial · Gage R&R · GD&T basics · Hazard vs risk vs harm · Hypothesis test selection · I-MR chart · Interaction plots · ISO 14971 · ISO 9001 clauses · Kaizen event planning · Kano model · Lean wastes (TIMWOODS) · Linear regression · Management review · MRB dispositions · MSA: bias, linearity & stability · NCR, CAPA & PA · Nonparametric tests · Normal distribution & z-table · One-way ANOVA · p-value explained · Pareto chart · PDCA vs DMAIC · Poka-yoke · QFD / House of Quality · Quality audits · Quality mgmt principles · Quality manual vs procedures · Quality plan vs control plan · Redundancy: parallel & standby · Reliability allocation · Bathtub curve & MTBF · Response surface methods · Risk-based thinking · Risk matrix vs FMEA · 7 mgmt & planning tools · Short-run SPC · Six Sigma certifications · Sigma level & DPMO · Six Sigma DMAIC · Supplier quality scorecards · Switching rules (sampling) · Taguchi loss function · Theory of constraints · Tolerance stackup · Traceability & lot control · Type I vs II error · Value stream mapping · Variables vs attributes · Weibull analysis · Western Electric rules · X-bar R vs X-bar S · CQE calculators · Study planner · Section quiz

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