Fishbone Diagram (Ishikawa Diagram): Meaning, 6M Method & Example


 In Previous blog we have learnt "What is a Pareto Chart and How It Helps in Quality Improvement". If you have not learnt go with below link - 

Fishbone Diagram (Ishikawa Diagram) – A Simple Guide to Finding Root Causes

The Fishbone Diagram is one of the 7 Quality Control (QC) Tools used to analyze problems in a structured way.

It is also called the Ishikawa Diagram, named after Kaoru Ishikawa, a Japanese quality expert who developed this method.

This tool helps teams identify the real root cause of a problem instead of just treating the symptoms.

The diagram looks like the skeleton of a fish, which is why it is commonly known as a Fishbone Diagram.

What is a Fishbone Diagram?

A Fishbone Diagram is a cause-and-effect analysis tool used to identify all possible reasons behind a problem.

It helps teams:

  • Identify possible causes of an issue
  • Organize causes into logical categories
  • Analyze the root cause, not just the visible problem
  • Improve product or process quality by eliminating the real defect

Instead of guessing why a problem happened, the fishbone diagram allows teams to systematically explore every possible cause.

The 6M Categories in Fishbone Analysis

In manufacturing and quality control, causes are usually grouped into six major categories, known as the 6M.

These categories help ensure that no possible cause is overlooked.

1. Man (Human Factors)

This category includes issues related to people involved in the process.

Examples:

  • Operator not properly trained
  • Improper handling of materials
  • Lack of concentration
  • Not following Standard Operating Procedures (SOP)

2. Machine

This category focuses on problems related to equipment or tools.

Examples:

  • Pick-and-place machine misalignment
  • Nozzle damage
  • Improper machine calibration
  • Lack of preventive maintenance

3. Method

Method refers to the process or procedure used to perform the work.

Examples:

  • Wrong process sequence
  • Incorrect soldering temperature
  • Poor or unclear work instructions

4. Measurement

This category relates to inspection, testing, and measuring systems.

Examples:

  • Incorrect inspection methods
  • Faulty measuring instruments
  • No periodic calibration of gauges

5. Material

Material includes raw materials, components, or consumables used in production.

Examples:

  • Poor quality PCB
  • Incorrect component specifications
  • Moisture in components

6. Environment

Environment refers to external conditions that affect the process.

Examples:

  • High humidity
  • Dust contamination
  • Poor lighting
  • Temperature variations

Example: Fishbone Diagram for PCB Mounting Issues

Let’s understand this with a simple example.

Problem Statement

PCB Mounting Issues such as:

  • Dry solder
  • Missing components

Another example could be molding defects in cabinets, such as:

  • Scratches
  • Burr formation

Using the Fishbone diagram, the team lists possible causes under each 6M category.

By analyzing all these possible causes, the team can identify the actual root cause of the defect.

Why is the Fishbone Diagram Important?

The Fishbone Diagram is widely used because it helps teams analyze problems logically instead of making assumptions.

It helps organizations to:

  • Identify the real root cause of defects
  • Take corrective and preventive actions
  • Prevent the same problem from happening again
  • Improve product quality
  • Reduce customer complaints
  • Encourage team brainstorming and collaboration

Key Elements of a Fishbone Diagram

A Fishbone diagram consists of four main parts.

1. Problem Statement (Head of the Fish)

The problem being analyzed is written at the right side, which represents the head of the fish.

Example:
"Increase in product rejection due to dry solder."

2. Main Categories (Main Bones)

The major bones represent the 6M categories:

  • Man - Operator training issue
  • Machine - Calibration problem
  • Method - Wrong work instruction
  • Measurement - Gauge error
  • Material - Poor supplier quality
  • Environment- Temperature/ Humidity
    Fishbone Diagram 6M root cause analysis example

3. Sub-Causes (Small Bones)

Each main category contains smaller branches that list the possible causes of the problem.

4. Root Cause

By analyzing the causes and applying Why-Why analysis or the 5 Whys, the team identifies the actual root cause of the issue.

How to Create a Fishbone Diagram (Step-by-Step)

Step 1: Define the Problem

Clearly define the issue you want to analyze.

Example:
"Customer complaints increased by 20% due to PCB soldering defects."

Step 2: Draw the Backbone

Draw a horizontal line pointing toward the problem statement.
This line represents the spine of the fish.

Step 3: Add the Major Categories

Add the 6M categories as branches from the main line.

Step 4: Brainstorm Possible Causes

Ask the question:

“Why does this problem happen?”

Add all possible causes under the relevant category.

Step 5: Use the 5 Whys

Continue asking “Why?” until the actual root cause becomes clear.

This helps prevent solving only the surface-level issue.

Common Mistakes When Using a Fishbone Diagram

Many teams make mistakes while using this tool. Some common ones include:

  • Creating the diagram but not performing Why-Why analysis
  • Not identifying the actual root cause
  • Missing important sub-causes
  • Not involving a cross-functional team
  • Not creating a corrective action plan
  • Not verifying whether the solution actually fixed the problem

Role of Fishbone Diagram in Quality Control

The Fishbone Diagram plays an important role in quality improvement and problem solving.

It helps organizations:

  • Identify root causes systematically
  • Improve process control
  • Support continuous improvement
  • Make decisions based on analysis instead of assumptions

Benefits of Using a Fishbone Diagram

Using a Fishbone Diagram provides several advantages:

✅ Helps solve the real problem, not just symptoms
✅ Improves product quality
✅ Reduces rejection and rework
✅ Minimizes customer complaints
✅ Saves operational cost
✅ Improves process stability

Conclusion

The Fishbone Diagram (Ishikawa Diagram) is one of the most powerful tools among the 7 QC Tools for root cause analysis.

By organizing possible causes under the 6M categories — Man, Machine, Method, Measurement, Material, and Environment — teams can systematically analyze problems and identify the real reason behind defects.

When used correctly along with Why-Why analysis, the Fishbone Diagram helps organizations take effective corrective actions, improve product quality, and prevent problems from recurring.

FAQ

What is a Fishbone Diagram used for?

A Fishbone Diagram is used to identify the root causes of a problem by categorizing possible causes under different groups such as Man, Machine, Method, Material, Measurement, and Environment.

Why is it called an Ishikawa Diagram?

It is called an Ishikawa Diagram because it was developed by Kaoru Ishikawa, a Japanese expert in quality management.

What are the 6M categories in a Fishbone Diagram?

The 6M categories are:

  • Man - Operator training issue
  • Machine - Calibration problem
  • Method - Wrong work instruction
  • Measurement - Gauge error
  • Material - Poor supplier quality
  • Environment- Temperature/ Humidity
If you are learning Quality Management and the 7 QC Tools, follow this blog series where we explain each tool with simple examples.

In the next blog, we will learn What is a Histogram? Meaning, Example and Uses in Quality Control.

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