Ishikawa 6M
Root cause analysis with 6M categories and expert scoring
Anatomy of an Ishikawa Diagram
The Ishikawa diagram (also called fishbone or cause-and-effect diagram) collects possible causes of a defined problem, structured by main categories. It is one of the standard tools of the Analyze phase and is typically developed in a team workshop.
Effect (head of the fish): The problem or deviation whose causes are sought — e.g. "Dimensional deviation > 0.1 mm", "Delivery delay > 2 days". Written on the far right as the "head".
Main bones (categories): The large diagonal lines — the classic 6M are: Man, Machine, Method, Material, Mother Nature (environment), Measurement. In services, 6P are often used (People, Process, Place, Product, Procedures, Policies).
Causes (sub-bones): Specific potential causes attached to each main bone. A cause can be broken down further into tertiary bones.
Depth (level of detail): Multi-level bones allow you to dig deeper by repeated "why?". Three levels are usually enough — anything beyond becomes confusing. For deeper root-cause analysis, use the 5-Why module.
The Ishikawa is not a proof tool — it is a structured collection of hypotheses. Which causes are actually relevant is verified afterwards with data (C&E matrix, hypothesis tests, DOE).
Approach
- Phrase the effect precisely — a concrete, measurable deviation, not a vague goal.
- Pick main categories: 6M for manufacturing, 6P for services, or your own fitting categories.
- In the workshop, give everyone a voice — brainstorming without judgment.
- Collect causes per category — first volume, then order.
- For important causes, drill down with "why?" until a concrete root becomes visible.
- Move the prioritized causes into the C&E matrix or directly into hypothesis tests.
Pitfalls
Effect too vague: "Quality is bad" cannot be analyzed. Formulate a measurable, bounded effect — then the causes also become concrete.
Symptoms instead of causes: "Machine fails" is an effect, not a cause. Drill down with "why does it fail?" until a technically or organizationally actionable cause becomes visible.
6M as compulsory exercise: If every category is forced full of causes, meaningless entries appear. Leave categories empty deliberately rather than inventing artificial "Man" material.
Hypotheses become facts: Whatever sits in the Ishikawa is a guess. Before deriving improvement actions, verify with data — otherwise lots of money and time go into phantom causes.
Built alone: Ishikawa lives from different perspectives. Operators, maintenance, quality, and engineering in the same workshop often surface causes nobody would have thought of alone.
Excessive depth: More than three levels become confusing. Anyone who wants to drill deeper should open a dedicated 5-Why module for the important branches.
Examples
This module ships with the following example datasets — load any of them in the app with a single click.
Available in the following cycles
- DMAIC: Analyze
- DMADV: Analyze
- 8D: D4 — Root Cause Analysis