Ishikawa Diagram; Fish-Bone Diagram; Cause-and-Effect Diagram

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.

The Ishikawa Diagram is usually constructed by the problem-solving team using the following basic steps:

1) prepare the basic framework of the Ishikawa Diagram on a large writing area, such as a whiteboard or a flipchart;

2) define the problem that needs to be addressed and describe it in clear and specific terms, then write this description in the problem box or fish head of the diagram;

3) finalize the cause categories of the major branches and write these at the tips of the major branches; if the members are all new to the Ishikawa Diagram and can't decide on which categories to write, use the 4 M's as categories;

4) conduct the brainstorming session using these basic brainstorming guidelines:

a) each participant will be asked one at a time to give a cause of the problem (only one input per turn!), saying 'Pass' if he or she can't think of any during his or her turn;

b) each cause identified will be 'hung' on the major branch of the category it belongs to; if it's the cause of another cause that's already on the diagram, then it must be 'hung' on the branch of the latter; if applicable, a cause may be placed on several branches;

c) the brainstorming session will continue until everyone says 'Pass'.

5) interpret the Ishikawa Diagram once it's finished.

There are many ways to interpret the Ishikawa Diagram. The fastest and simplest way to do it is for the group to choose the top five causes on the diagram and rank them, using their collective knowledge and any data available. The selection of the major causes may be done by voting or any other process that allows the group to agree on the ranking. The selected causes are then encircled on the diagram, with their ranks written beside them. The team may then investigate these causes further and use problem-solving techniques to eliminate their occurrences.

Figure 2. Example of a simple but finished Ishikawa Diagram

See Also: Pareto Chart; Brainstorming

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