It is simply a diagram that shows the causes of a certain event. It was first used in the 1960s, and is considered one of the seven basic tools of quality management, along with the histogram, Pareto chart, check sheet, control chart, flowchart, and scatter diagram. See Quality Management Glossary.
It is known as a fishbone diagram because of its shape, similar to the side view of a fish skeleton.
Causes in the diagram are often based around a certain category or set of causes, such as the 6 M's, 8 P's or 4 S's described below. Cause-and-effect diagrams can reveal key relationships among various variables, and the possible causes provide additional insight into process behaviour.
Causes in a typical diagram are normally arranged into categories, the main ones of which are:
The 6 M's
Machine, Method, Materials, Measurement, Man and Mother Nature (Environment) (recommended for manufacturing industry).
Note: a more modern selection of categories used in manufacturing includes Equipment, Process, People, Materials, Environment, and Management.
The 8 P's
Price, Promotion, People, Processes, Place / Plant, Policies, Procedures & Product (or Service) (recommended for administration and service industry).
The 4 S's
Surroundings, Suppliers, Systems, Skills (recommended for service industry).
It can also be used in connection with the Neuro-linguistic programming model of the Neurological Levels created by Robert Dilts: with Identity, Beliefs and Values, Capability, Behaviour, Environment.
A common use of the Ishikawa diagram is in product design, to identify desirable factors leading to an overall effect. Mazda Motors famously used an Ishikawa diagram in the development of the Miata sports car, where the required result was "Jinba Ittai" or "Horse and Rider as One". The main causes included such aspects as "touch" and "braking" with the lesser causes including highly granular factors such as "50/50 weight distribution" and "able to rest elbow on top of driver's door". Every factor identified in the diagram was included in the final design.
Appearance of Fishbone Diagrams
Most Fishbone diagrams have a box at the right hand side in which is written the effect that is to be examined. The main body of the diagram is a horizontal line from which stem the general causes, represented as "bones". These are drawn towards the left-hand side of the paper and are each labeled with the causes to be investigated, often brainstormed beforehand and based on the major causes listed above.
Off each of the large bones there may be smaller bones highlighting more specific aspects of a certain cause, and sometimes there may be a third level of bones or more. These can be found using the '5 Whys' technique.
When the most probable causes have been identified, they are written in the box along with the original effect. The more populated bones generally outline more influential factors, with the opposite applying to bones with fewer "branches". Further analysis of the diagram can be achieved with a Pareto chart.