Cause-effect diagrams are a useful technique that can be used to improve quality. Glenford J. Myers wrote that “a weakness of boundary value analysis and equivalence partitions is that they do not explore combinations of input circumstances”[1]. A technique that can be used to explore and describe combinations of inputs to an issue is a cause-effect diagram.
A type of cause-effect graph I have found useful is the Ishikawa or Fishbone Diagram. The diagram takes its name from Professor Kaoru Ishikawa who invented the diagram[2]. It is also known as a fishbone diagram because it looks like a fish’s bones. An example diagram is shown above. The effect that you are concerned about is shown in the diagram on the fish’s head. The effect shown in the diagram could be a bug or a production incident that you want to analyse. The causes of the effect are shown on the fish bones, and sub-causes are shown as branches of the fish bones.
The diagrams were named Ishikawa Diagrams by Joseph Juran [3], and are an example of how he and W. Edwards Deming brought ideas from Japan to North America and Europe.
Ishikawa categorised the causes of the issue, that is being analysed, into five groups called the Five Ms:
- Material. Everything that is consumed by the project such as raw materials.
- Method. Existing procedures and information flow.
- Mother Nature. The environment and context.
- Machine. The necessary equipment for the project.
- (Hu)manpower. The human resources involved in the project.
An Ishikawa diagram can be used to find the root cause of a production incident or another complex issue. A root cause of the effect being analysed should be written on a bone in the diagram, and each root cause should also be analysed to find its root cause. The root cause of one of the causes should be written as a branch to the bone.
Ishikawa diagrams can be used to improve quality as they can be used to identify the causes of an issue, and countermeasures can then be identified to remove the causes of the issue.
The diagram may be created when you are working with a team or when you are working alone. The team can create a diagram by suggesting causes which are then added to the diagram. To help find the causes of the effect being analysed, the Five Ms can be used as prompts to consider the factors they represent. An example would be to consider material, this could be used to consider all the third-party components used and (Hu)manpower could be used to consider what training people working on the project have had and need.
An advantage of using an Ishikawa diagram is that it is visual and so it is easy to review by looking over the causes identified on the diagram. This can make it easy to work iteratively on finding the causes of an issue. The diagram is easy to share as it is one diagram rather than a long document. Another advantage is that there is also no need for notes that have to be written up after a team creates a diagram as the diagram explains itself.
Two disadvantages of Ishikawa diagrams are that they can be unfamiliar and may not look like diagrams that people are used to using. The success of teams and individuals, who have not used Ishikawa diagrams previously, relies on their wanting to learn how to use fishbone diagrams. If you want the team to use Ishikawa diagrams then it is your role to persuade them to do so.
Learning how to use an Ishikawa diagram is a useful skill. If a team or individual learns how to use Ishikawa diagrams, they will likely use them again later in their career. Ishikawa created fishbone diagrams in 1943[2], so they have been in use for a long time and it is reasonable to expect them to be used for the foreseeable future. Fishbone diagrams are useful and are used widely. Janet Gregory and Lisa Crispin suggest using Ishikawa diagrams to visualise your way of thinking and to help with identifying risks[5]. Masaaki Imai lists fishbone diagrams as one of the Seven Statistical Tools involved in Kaizen problem-solving[4].
There are many templates available for creating Ishikawa diagrams, to find them just google “Ishikawa diagram templates”. You should be able to find a template for an application that you use.
It would be great to hear of your experiences of using Ishikawa diagrams to improve quality
References
[1] Glenford J. Myers The Art of Software Testing
[2]. 50MINUTES series Ishikawa Diagram: Anticipate and solve problems within your business (Management & Marketing)
[3] Kaoru Ishikawa What is Total Quality Control – The Japanese Way
[4] Masaaki Imai Kaizen the Key to Japan’s Competitive Success
[5] Janet Gregory and Lisa Crispin More Agile Testing
Further Reading
- Dr Joseph Juran Juran’s Quality Handbook (5th Edition) ( see the section on The Quality Management Process)
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