A cause and effect diagram is a visual tool that maps all potential causes of a specific problem into structured categories, displayed as branches off a horizontal spine that leads to the problem statement. The diagram’s shape resembles a fish skeleton — the problem sits at the fish’s head, the major cause categories form the large bones, and sub-causes branch off each major bone.
It is one of the Seven Basic Quality Tools in Six Sigma and the primary brainstorming tool of the Analyze phase of DMAIC. Its purpose is not to identify the confirmed root cause — that requires data. Its purpose is to systematically generate every plausible hypothesis the team should test.
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Meaning of Cause and Effect Diagram
A cause and effect diagram — also called a fishbone diagram or Ishikawa diagram — is a visual root cause analysis tool that displays the potential causes of a specific problem in a structured, categorized format.
The problem appears at the right end (the fish’s head). Major cause categories form the large diagonal bones. Sub-causes branch off each category bone. The diagram was created by Professor Kaoru Ishikawa in 1943 at the Kawasaki Steel Works to help engineers think systematically about multiple interacting causes.
Joseph Juran formally named it the “Ishikawa diagram” in the 1962 edition of his Quality Control Handbook. In Six Sigma DMAIC, it is the primary brainstorming tool of the Analyze phase, used to generate candidate causes (Xs) before they are statistically confirmed.
Key Takeaways
- A cause and effect diagram maps all potential causes of a problem into structured categories, displayed as a fish skeleton with the problem at the head and cause branches as bones.
- It has three names used interchangeably: cause and effect diagram (its function), fishbone diagram (its appearance), and Ishikawa diagram (its creator).
- Professor Kaoru Ishikawa created the diagram in 1943 at the Kawasaki Steel Works to help engineers organize complex, interacting factors affecting a problem.
- Joseph Juran formally named it the “Ishikawa diagram” in his 1962 Quality Control Handbook in recognition of its inventor.
- The diagram is one of the Seven Basic Quality Tools of quality management. The other six are the check sheet, control chart, histogram, Pareto chart, scatter diagram, and stratification (or run chart).
- The standard manufacturing cause categories are the 6Ms: Man (People), Machine, Method, Material, Measurement, and Mother Nature (Environment). Service industries often use the 5Ps: People, Process, Policies, Plant, and Patrons.
- The cause and effect diagram belongs in the Analyze phase of DMAIC. It generates hypotheses. Statistical tools (hypothesis tests, regression analysis) then confirm or reject each hypothesis.
- The diagram does not identify the root cause by itself. After completing the diagram, the team uses a tool like the 5 Whys to drill deeper into likely causes.
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What Is a Cause and Effect Diagram?
A cause and effect diagram is a structured graphical tool that lists all possible causes of a problem and shows their relationships to the central effect being studied. The cause and effect relationship is the core concept: the “effect” is the problem — the measurable output that is not meeting requirements. The “causes” are the input factors and process conditions that may be producing that problem.
The diagram makes the brainstorming process systematic rather than random. Instead of allowing a team to generate causes in any direction, the diagram organizes them into major categories. Every team member thinks about causes within each category, forcing coverage of all operational domains rather than just the ones that come to mind first.
The Kaizen Institute confirms this purpose directly: “Developed within the context of quality management and control, this diagram is widely used in methodologies such as Lean, Kaizen, and Six Sigma to drive continuous improvement.”
The Taylor and Francis academic reference summarizes the output clearly: “The Cause and Effect diagram is a structured graphical diagram listing all possible causes and their relationship with the main effect or problem.”
Also See: Cause and Effect Matrix Online Training (Complete with Certification)
The Origin of the Cause and Effect Diagram
The cause and effect diagram has a specific, verifiable history. Multiple authoritative sources — the Taylor and Francis academic database, the ASQ, the Kaizen Institute, and the Six Sigma Study Guide — confirm the same account.
1943 — Kawasaki Steel Works. Professor Kaoru Ishikawa, a Japanese quality control expert and statistician at the University of Tokyo, first used the diagram at the Kawasaki Steel Works. He used it to explain to a group of engineers how various work factors could be sorted and related to a problem. The goal was to help teams avoid jumping to the first plausible cause and instead consider the full system of interacting factors.
At that time in many Japanese manufacturing environments, quality problems were often blamed on individual workers. Ishikawa designed the diagram to shift attention away from people and onto the system of causes — a principle that aligns directly with Six Sigma’s focus on process improvement rather than individual blame.
1962 — Joseph Juran names it. Joseph Juran, the American quality management pioneer, formally named the tool the “Ishikawa diagram” in the 1962 edition of his Quality Control Handbook. This naming recognized Ishikawa’s contribution and helped establish the diagram’s place in the global quality management canon.
1960s — Toyota quality circles. The diagram spread widely through Toyota’s quality circles in the 1960s, where it became a standard tool for team-based root cause analysis. This adoption embedded it in the global quality management vocabulary and connected it to the manufacturing world that would later develop Lean and Six Sigma.
The diagram is now one of the Seven Basic Quality Tools, a set of foundational quality techniques identified and promoted by Ishikawa himself.
The Three Names Explained
The same tool carries three different names. Understanding what each name references prevents confusion when reading training materials, exam prep content, or project documentation.
| Name | Why It Is Called This |
| Cause and effect diagram | Describes the tool’s function: it shows causes and their relationship to an effect |
| Fishbone diagram | Describes the tool’s appearance: it resembles a fish skeleton |
| Ishikawa diagram | Honors the tool’s creator: Professor Kaoru Ishikawa |
All three names describe exactly the same tool. Training materials, certification exams, and quality references use all three interchangeably.
The 6M Cause Categories for Manufacturing
Ishikawa introduced generic cause category labels to give teams a consistent starting framework. He also encouraged teams to adapt the labels to communicate more clearly for their specific context — as the ASQ notes, “he also encouraged creativity in naming these categories.”
The most widely used category set in manufacturing is the 6Ms:
1. Man (People) All human-related causes: operator skill, training, fatigue, experience level, communication errors, and variations in how different people perform the same task.
2. Machine All equipment-related causes: equipment age, maintenance status, tool wear, calibration, setup accuracy, and machine capability.
3. Method All process-related causes: standard operating procedures, work instructions, process sequence, inspection methods, and whether the defined method is actually being followed.
4. Material All input material causes: raw material variation, supplier differences between lots, incoming quality, material specifications, and storage conditions.
5. Measurement All measurement system causes: gauge calibration, measurement technique, Gauge R&R, the measurement instrument’s resolution relative to the tolerance, and how results are recorded.
6. Mother Nature (Environment) All environmental causes: temperature, humidity, vibration, lighting, contamination, and any ambient condition that varies and could affect the output.
Every cause on the diagram fits within one of these six categories. If a team generates a cause that does not fit clearly into any category, that may signal either that the category labels need to be adapted for their specific process or that the cause needs to be described more precisely.
Also See: What Is Green Lean Six Sigma (GLSS)? Definition, Tools, and Real Results
Cause Categories for Service Environments: The 5Ps
Manufacturing and service processes share some cause categories but differ in others. Service organizations often use the 5Ps instead of the 6Ms:
1. People — Equivalent to Man in the 6M framework 2. Process — The sequence of steps and procedures used to deliver the service 3. Policies — Organizational rules, regulations, and governance that affect how work is done 4. Plant (Environment) — The physical or digital workspace and its conditions 5. Patrons (Customers) — How customer behavior, inputs, or variability affect the output
Healthcare, banking, logistics, and software service organizations commonly adapt the 5Ps to their specific context, sometimes replacing “Plant” with “Technology” or adding a sixth “P” for “Partners” or “Providers.”
How to Build a Cause and Effect Diagram: Step-by-Step
The following steps describe the complete process for building a cause and effect diagram. The typical team size for this exercise is four to eight people with direct knowledge of the process being investigated.
Step 1: Define the effect (problem statement). Write the problem statement in a box on the right side of the page. This is the fish’s head. The statement must be specific and measurable. “High defect rate” is too vague. “Valve diameter nonconformance rate of 4.2% over the past 30 days, against a target of below 1%” is a usable problem statement. A specific problem statement keeps the brainstorming focused.
Step 2: Draw the spine and major bones. Draw a horizontal arrow pointing toward the problem statement box. This is the fish’s spine. Draw the major category bones as diagonal arrows branching off the spine, three on each side. Label each bone with one of the 6M (or 5P) categories. Space the bones evenly so that each category has room for multiple sub-causes.
Step 3: Brainstorm causes within each category. The team works through each category systematically. For each category, they ask: “What in this category could cause the problem we identified?” Write each suggested cause as a branch off the relevant category bone.
Encourage all suggestions without judgment during the brainstorming phase. Evaluation comes later. A cause that seems unlikely to one person may connect to a real root cause that another person recognizes.
Step 4: Add sub-causes. For each cause written on a category branch, ask “What causes this?” and add the answer as a sub-branch. This is the same logic as the 5 Whys — peeling back each layer to find what lies beneath the surface cause.
Sub-causes turn a list of symptoms into a more diagnostic map. “Tool wear” on the Machine bone might have sub-causes of “exceeded replacement cycle” and “no tool wear monitoring in place.”
Step 5: Review for completeness. After brainstorming, review each category. Did the team generate at least two or three causes in each? If one category is empty or nearly empty, that is a signal to spend more time thinking about it — not a signal that the category is irrelevant.
Step 6: Prioritize the most likely causes. The completed diagram may contain twenty to fifty candidate causes. The team cannot statistically test all of them. Use multi-voting, the Pareto principle, or team knowledge to identify the five to ten causes that are most likely to be contributing significantly to the problem. Circle or highlight these for the next phase of investigation.
Step 7: Validate causes with data. The diagram itself does not prove causation. It generates hypotheses. Each prioritized cause must be tested with process data using statistical tools — hypothesis tests, correlation analysis, regression, or a designed experiment — to confirm whether it actually drives the output.
The Cause and Effect Diagram and the 5 Whys
The cause and effect diagram and the 5 Whys are complementary tools, not competing ones. They are used in sequence.
The cause and effect diagram produces a wide, comprehensive map of all potential causes across all major categories. It ensures no category is overlooked. The 5 Whys then drill vertically into the most likely causes on the diagram, asking “Why?” repeatedly until the team reaches the fundamental root cause rather than a surface symptom.
The AI Gro Process Excellence blog summarizes this combination: “In DMAIC, the fish head is your measurable Y and the bones are the candidate Xs you must test.”
The two tools together — breadth from the fishbone, depth from the 5 Whys — provide the hypothesis list that statistical testing in the Analyze phase then validates or eliminates.
The Cause and Effect Diagram in the DMAIC Analyze Phase
The cause and effect diagram belongs specifically to the Analyze phase of DMAIC. At this point in the project:
- The Define phase has established the problem statement and project Y.
- The Measure phase has confirmed the baseline performance and validated the measurement system.
- The Analyze phase must now identify the root causes driving the gap between current and target performance.
The fishbone diagram is the primary structured brainstorming tool for this root cause identification work. Multiple authoritative sources confirm this placement. The Six Sigma Study Guide states: “By and large, Fishbone diagrams are used to identify the root causes of a problem in the ‘Analyze’ phase of Six Sigma’s DMAIC.”
The Symestic manufacturing intelligence platform confirms: “In DMAIC, the Ishikawa diagram is the primary tool of the Analyse phase.”
It is critical to understand what the diagram does and does not do within DMAIC:
What it does: Generates a comprehensive, categorized list of candidate causes that the team should investigate with data.
What it does not do: Confirm that any cause is actually driving the problem. Confirmation requires data, statistical testing, and analysis.
Teams that confuse hypothesis generation with root cause confirmation make one of the most costly DMAIC mistakes: they implement solutions targeting causes that have never been validated, waste Improve phase resources, and return months later when the problem has not resolved.
| DMAIC Phase | Role of the Cause and Effect Diagram |
| Define | Not used; Define establishes the problem statement that the diagram will later analyze |
| Measure | Not typically used; Measure collects baseline data on the confirmed Y |
| Analyze | Primary use: generates structured hypothesis list of candidate Xs to test |
| Improve | Solutions in the Improve phase target the Xs confirmed during Analyze — the fishbone informed which Xs to confirm |
| Control | Not used; Control monitors the Xs confirmed during Analyze |
Frequently Asked Questions: Cause and Effect Diagram
Q: What is a cause and effect diagram?
A: A cause and effect diagram is a visual root cause analysis tool that maps all potential causes of a specific problem into structured categories. The problem appears at the right end of the diagram (the fish’s head). Major cause categories form the diagonal bones, and sub-causes branch off each category. It is also called a fishbone diagram (for its appearance) and an Ishikawa diagram (for its creator). It is one of the Seven Basic Quality Tools and the primary brainstorming tool of the Analyze phase in Six Sigma DMAIC.
Q: Who invented the cause and effect diagram?
A: Professor Kaoru Ishikawa invented the diagram in 1943 at the Kawasaki Steel Works, where he used it to help engineers organize the complex, interacting factors affecting a problem. Joseph Juran formally named it the “Ishikawa diagram” in the 1962 edition of his Quality Control Handbook. The diagram spread widely through Toyota’s quality circles in the 1960s and became one of the Seven Basic Quality Tools of quality management.
Q: What are the 6M cause categories?
A: The 6Ms are the standard cause categories for manufacturing environments: Man (People), Machine, Method, Material, Measurement, and Mother Nature (Environment). Every potential cause of a problem fits within one of these six categories. ASQ confirms that Ishikawa introduced the 6Ms as generic starting labels but encouraged teams to adapt them to communicate more clearly for their specific process context.
Q: What is the difference between a fishbone diagram and an Ishikawa diagram?
A: There is no difference. Both names describe exactly the same tool. Fishbone diagram refers to the tool’s visual appearance, which resembles a fish skeleton. Ishikawa diagram honors its creator, Professor Kaoru Ishikawa. Cause and effect diagram describes its function. All three names are used interchangeably in Six Sigma training materials and certification exams.
Q: Does the cause and effect diagram identify the root cause?
A: No. The cause and effect diagram generates a structured list of candidate causes — hypotheses that the team should test. It does not confirm which causes are actually driving the problem. Confirming the root cause requires data collection and statistical testing using tools such as hypothesis tests, correlation analysis, or regression. The diagram is a brainstorming tool; statistical analysis provides the confirmation.
Q: Where does the cause and effect diagram fit in the DMAIC framework?
A: The cause and effect diagram belongs in the Analyze phase of DMAIC. The Define phase establishes the problem statement. The Measure phase collects baseline data on the Y variable. The Analyze phase then uses the fishbone diagram to generate all candidate causes (Xs) before using statistical tools to confirm which ones actually drive the Y. The fishbone diagram’s output — a prioritized list of candidate causes — directly determines which variables the team tests statistically.
Q: How is the cause and effect diagram different from the 5 Whys?
A: The cause and effect diagram produces a wide map of all potential causes across all major categories. The 5 Whys drill vertically into specific causes, asking “Why?” repeatedly to reach the fundamental root cause beneath a surface symptom. The two tools are used in sequence: the fishbone provides breadth and ensures no category is overlooked; the 5 Whys add depth by peeling back layers within the most likely causes the diagram identifies.
Cause and Effect Diagram Training in Six Sigma
The cause and effect diagram is introduced at the Yellow Belt level as a foundational quality tool and applied extensively at the Green Belt and Black Belt levels within DMAIC projects. It is one of the Seven Basic Quality Tools tested on the IASSC certification exam at all belt levels.
Practitioners who know only that the diagram exists — but not how to facilitate an effective brainstorming session, how to prevent the diagram from becoming a list of symptoms rather than causes, and how to connect its outputs to statistical hypothesis testing — will build diagrams that look complete but produce no actionable analytical direction.
At Six Sigma Development Solutions, the cause and effect diagram is taught as an applied Analyze phase skill across our Yellow Belt, Green Belt, and Black Belt training programs. Practitioners learn to facilitate a structured brainstorm, build a complete six-category fishbone, prioritize hypotheses, and connect the diagram’s output to Minitab-based hypothesis testing.
We offer training in three formats:
- Onsite training — delivered at your facility, with a live fishbone brainstorming session on a real process problem during class.
- Live virtual training — instructor-led sessions online covering the cause and effect diagram, 5 Whys, and their integration into the full DMAIC Analyze phase.
- Online training — self-paced Yellow Belt, Green Belt, and Black Belt certification programs covering all IASSC-testable root cause analysis tools.
Explore our Six Sigma training programs or contact our team to find the right program for your goals.
About Six Sigma Development Solutions, Inc.
Six Sigma Development Solutions, Inc. offers onsite, public, and virtual Lean Six Sigma certification training. We are an Accredited Training Organization by the IASSC (International Association of Six Sigma Certification). We offer Lean Six Sigma Green Belt, Black Belt, and Yellow Belt, as well as LEAN certifications.
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