| Category: | One of the Seven Basic Tools of Quality |
| Describer: | Kaoru Ishikawa |
| Purpose: | To break down (in successive layers of detail) root causes that potentially contribute to a particular effect |
Ishikawa diagrams (also called fishbone diagrams, herringbone diagrams, cause-and-effect diagrams) are causal diagrams created by Kaoru Ishikawa that show the potential causes of a specific event.[1]
Common uses of the Ishikawa diagram are product design and quality defect prevention to identify potential factors causing an overall effect. Each cause or reason for imperfection is a source of variation. Causes are usually grouped into major categories to identify and classify these sources of variation.
The defect, or the problem to be solved, is shown as the fish's head, facing to the right, with the causes extending to the left as fishbones; the ribs branch off the backbone for major causes, with sub-branches for root-causes, to as many levels as required.[2]
Ishikawa diagrams were popularized in the 1960s by Kaoru Ishikawa,[3] who pioneered quality management processes in the Kawasaki shipyards, and in the process became one of the founding fathers of modern management.
The basic concept was first used in the 1920s, and is considered one of the seven basic tools of quality control.[4] It is known as a fishbone diagram because of its shape, similar to the side view of a fish skeleton.
Mazda Motors famously used an Ishikawa diagram in the development of the Miata (MX5) sports car.[5]
Root-cause analysis is intended to reveal key relationships among various variables, and the possible causes provide additional insight into process behavior. It shows high-level causes that lead to the problem encountered by providing a snapshot of the current situation.
There can be confusion about the relationships between problems, causes, symptoms and effects. Smith[6] highlights this and the common question “Is that a problem or a symptom?” which mistakenly presumes that problems and symptoms are mutually exclusive categories. A problem is a situation that bears improvement; a symptom is the effect of a cause: a situation can be both a problem and a symptom.
At a practical level, a cause is whatever is responsible for, or explains, an effect - a factor "whose presence makes a critical difference to the occurrence of an outcome".[7]
The causes emerge by analysis, often through brainstorming sessions, and are grouped into categories on the main branches off the fishbone. To help structure the approach, the categories are often selected from one of the common models shown below, but may emerge as something unique to the application in a specific case.
Each potential cause is traced back to find the root cause, often using the 5 Whys technique.[8]
Typical categories include:
See also: 5M model. Originating with lean manufacturing and the Toyota Production System, the 5 Ms is one of the most common frameworks for root-cause analysis:[9]
These have been expanded by some to include an additional three, and are referred to as the 8 Ms:[10]
See also: Marketing mix. This common model for identifying crucial attributes for planning in product marketing is often also used in root-cause analysis as categories for the Ishikawa diagram:
An alternative used for service industries, uses four categories of possible cause:[11]