Ishikawa diagrams are also referred to as fish-bone (due to their shape and structure), or cause and effect diagrams. A fish-bone diagram is a commonly used quality tool and
method with the aim of finding the root cause of a problem.
The basic theory and principle of the fish-bone diagram was established in the '60s, by Kaoru Ishikawa, a Japanese professor and
innovator, who deeply participated in Japan’s
initiatives after the 2nd World War, with the support of Deming and Juran.
Dr. Ishikawa found out, that the causes of fundamental problems can be summarized more easily if they are categorized in a pre-defined structure. As each defect is a result of
one or the correlation of more defect causes, the Ishikawa diagram is a logic tool that is able to visualize the defect categories, and even their interaction as well.
The setup of an Ishikawa analysis starts with the clear structuring of potential root causes
Ishikawa is a team work, best if used during brainstorming.
The structure of the fish-bone is simple:
- 1) The head of the fish is always the precisely defined fundamental problem (effect of the failure). Important as always: the precise problem description is inevitable.
- 2) After stating the fundamental problem, we go along the spine of the fish. The spine will be the manifold link between the cause categories and the fundamental problem.
- 3) Several bones, ribs are attached to the spine, these will be the categories or groups of failure causes (see 7M below). The team collects all influencing factors that has any relation to the fundamental
- 4) The best in the Ishikawa diagram is that further bones (sub-causes) can be attached to the upper level bones, and so on, so the tool flawlessly visualizes the cause-effect relationship.
- 5) As soon as all possible root causes are filled into the fish-bone structure, the team has to include those ones that could cause, and exclude the ones that could not cause the particular failure.
This inclusion and exclusion is not based simply on theory, but happens with proofs and evidences by the verification and reproduction of the failure mode.
The major cause categories (7M) are considered during the elaboration of an Ishikawa analysis:
- Man: all failures that can be linked to the people and human factors.
- Method: means the causes that are originated from the production method, procedures, regulations, etc.
- Machine: the causes coming from the machinery and equipment.
- Material: all raw materials and sub-assemblies that are used for producing the final good.
- Environment (milieu): the failure causes, coming from the environment specific factors, such as time, shift, temperature, humidity, etc.
- Measurement: our measurement, data collection and control methods that might be having effect or impact on our production. This is a category, which is usually forgotten, but very important.
- Management: organizational and management related causes.
The structure of the Fish-Bone diagram (Ishikawa) (Source: qMindset.com)
On top of the 7M cause categories, some organizations also consider the effects of Maintenance, as a separated root cause category.
Benefits of Ishikawa:
- Exploration of possible root causes.
- With its predefined 5M or 7M structure, we can easily see if we missed to consider a failure cause category (e.g. "we have not evaluated the possible causes originated from the materials and the
- The fish-bone diagram is a visual tool, so gives a much more transparent view, than many other methodologies.
- Helps to pre-filter the major root cause possibilities for further deep analysis.
- Can easily be integrated into a deeper analysis framework (like 6Sigma), and can be attached to other quality tools, such as
Ishikawa is not only tool that can be used for reactive problem solving
, but also for a proactive
defect prevention analysis during the product- and process design.
For deeper technical problems, the joint use of Ishikawa and 5Why is very effective. After collecting the (most-likely) possible root causes with Ishikawa, the 5Whys methodology
can be used for deeper understanding of all major causes that were not excluded.