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The Ishikawa Diagram
The Ishikawa Diagram, also known as the Fishbone Diagram or the Cause-and-Effect Diagram, is a tool used for systematically identifying and presenting all the possible causes of a particular problem in graphical format. The possible causes are presented at various levels of detail in connected branches, with the level of detail increasing as the branch goes outward, i.e., an outer branch is a cause of the inner branch it is attached to. Thus, the outermost branches usually indicate the root causes of the problem.
The Ishikawa Diagram resembles a fishbone (hence the alternative name "Fishbone Diagram") - it has a box (the 'fish head') that contains the statement of the problem at one end of the diagram. From this box originates the main branch (the 'fish spine') of the diagram. Sticking out of this main branch are major branches that categorize the causes according to their nature.
In semiconductor manufacturing, 4 major branches are often used by beginners, referred to as the '4 M's', corresponding to 'Man', 'Machine', 'Materials', and 'Methods'. Sometimes 5 branches are used ('5 M's'), with the fifth branch standing for 'Measurement', or even 'M-vironment.' These 'M's' or problem cause categories are used to classify each cause identified for easier analysis of data. Of course, one is not constrained to use these categories in a fishbone diagram.
Experienced users of the diagram add more branches and/or use different categories, depending on what would be more effective in dealing with the problem. Figure 1 shows the basic framework of an Ishikawa Diagram.
Figure 1. The Basic '4 M's' Framework of an Ishikawa Diagram
The Ishikawa Diagram is employed by a problem-solving team as a tool for collating all inputs (as to what are the causes of the problem they're addressing) systematically and graphically, with the inputs usually coming from a brainstorming session. It enables the team to focus on why the problem occurs, and not on the history or symptoms of the problem, or other topics that digress from the intent of the session. It also displays a real-time 'snap-shot' of the collective inputs of the team as it is updated.
See Also: Pareto Chart; Brainstorming
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