Ishikawa diagram

What it means

An Ishikawa diagram, also called a fishbone or cause-and-effect diagram, is a tool for systematically searching for the possible causes of a problem. The problem is written in the fish's head and the main categories of causes form the bones, to which the team adds specific factors.

It is named after the Japanese quality expert Kaoru Ishikawa. In manufacturing, six categories known as the 6Ms are used: people, methods, machines, materials, measurement and environment. In services and administration the categories are adapted, for example to processes, systems, communication and customer.

The diagram does not find the cause by itself. What it does is stop the team from settling on the first obvious answer during root cause analysis and make it look at the problem from several angles. The result is a list of hypotheses to be tested with data and observation.

How it works in practice

The method is simple: the team defines the problem precisely, such as a rise in complaints about a particular product, draws the bones and looks for possible causes in each category. For the most likely branches it then keeps asking, ideally with the 5 Whys technique.

The diagram is a standard part of handling customer complaints, especially in the automotive industry, where the customer requires an 8D report. In its fourth step the supplier shows how it searched for the root cause, and the Ishikawa diagram is the most common tool for that.

When there are many possible causes, it helps to add data and rank causes by impact, for example using the Pareto principle. That way the team does not spend equal time on every idea but concentrates on those that explain most of the defects.

What to watch out for

A common mistake is a problem stated too broadly, such as poor quality. The diagram then covers everything and helps with nothing. Define the problem in measurable terms: what exactly, where, since when and to what extent, ideally with figures from complaints or inspections.

The diagram should not be used to find someone to blame. If the people category is reduced to a careless worker, the real cause in the process remains. Good practice is to state for each cause how it will be verified and what preventive action will follow, for example an update to the FMEA.