Using 5 Whys and Fishbone (Ishikawa) Diagram - Root Cause Analysis (RCA) in Lean Six Sigma

 

Root Cause Analysis (RCA) in Lean Six Sigma

Using 5 Whys and Fishbone (Ishikawa) Diagram

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What is Root Cause Analysis (RCA)?

Root Cause Analysis is a structured problem-solving approach used in Lean Six Sigma to identify the real cause of a problem—not just its symptoms.

Goal: Fix the problem once and permanently, not repeatedly.

RCA is mainly used in the Analyze phase of DMAIC.

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Why RCA is Critical in Lean Six Sigma

• Prevents recurring defects

• Avoids “quick fixes”

• Reduces waste and rework

• Enables sustainable improvement

• Supports data-driven decisions

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Two Most Powerful RCA Tools

Lean Six Sigma commonly uses:

1. 5 Whys

2. Fishbone (Cause-and-Effect) Diagram

They are often used together.

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1️⃣ 5 Whys Technique

What is 5 Whys?

The 5 Whys is a simple questioning technique where you repeatedly ask “Why?” until the root cause is revealed.

📌 Usually 5 times—but it can be 3 or 7, depending on the problem.

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5 Whys – Example

Problem Statement

❌ Customer received wrong invoice

Why?	Answer
1️⃣ Why was the invoice wrong?	Incorrect data was entered
2️⃣ Why was incorrect data entered?	Operator selected wrong customer
3️⃣ Why did operator select wrong customer?	Customer names look similar
4️⃣ Why do names look similar?	System does not show unique ID clearly
5️⃣ Why doesn’t system show unique ID?	Design requirement was missed

✅ Root Cause

👉 System design does not enforce unique customer identification

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When to Use 5 Whys

✔ Simple problems
✔ One main cause suspected
✔ Small teams
✔ Fast analysis

Limitations

❌ Can be subjective
❌ Depends on facilitator skill
❌ Weak for complex problems

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2️⃣ Fishbone Diagram (Ishikawa)

What is a Fishbone Diagram?

The Fishbone Diagram visually maps all possible causes of a problem under logical categories.

It answers:

What are all the possible reasons this problem could occur?

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Common Fishbone Categories (6M)

Manufacturing (6M)

• Man – People, skills, training

• Machine – Equipment, tools

• Method – Process, SOP

• Material – Raw materials

• Measurement – Data, metrics

• Mother Nature – Environment

Service / IT (often adapted)

• People

• Process

• System

• Policy

• Data

• Environment

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Fishbone – Example

Problem

❌ High order delivery delay

Possible causes identified:

• People: New staff, inadequate training

• Process: No standard delivery workflow

• System: ERP downtime

• Measurement: No delivery SLA tracking

• Environment: Traffic, weather

👉 After brainstorming, data analysis confirms:

• 70% delays due to missing standard process

✅ Root Cause

👉 Lack of standardized delivery process

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How 5 Whys and Fishbone Work Together (Best Practice)

Step	Tool
Brainstorm all possible causes	Fishbone
Narrow down key causes	Data analysis
Drill into true cause	5 Whys
Validate cause	Evidence
Fix permanently	Improve phase

📌 Fishbone = Wide view
📌 5 Whys = Deep dive

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Common Mistakes to Avoid ❌

• Jumping to solutions too early

• Stopping at symptoms

• Blaming people instead of process

• Not validating causes with data

• Treating assumptions as facts

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Lean Six Sigma Golden Rule ⭐

If you remove the root cause, the problem will not return.
If the problem returns, the root cause was not removed.

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Short Summary

Root Cause Analysis in Lean Six Sigma is used in the Analyze phase to identify the true cause of defects.
Fishbone diagrams identify all possible causes, while 5 Whys drills down to the actual root cause for permanent improvement.

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