The Psychology of Quality and More

A Toolbook for Quality Improvement and Problem Solving (contents)

Cause-Effect Diagram: How to understand it

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When to use it | How to understand it | Example | How to do it | Practical variations

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How to understand it

Solutions to problems are often not easy to find, and those that at first appear to be obvious may address only symptoms rather than the true cause of the problem. Identifying causes as an intermediate step makes solutions both easier to find and also more likely to address the problem fully.

Causes tend to appear in chains (Fig. 1), where one cause is caused by another, and so on. Thus an accident may be caused by a puncture, which is caused by a weak tire wall, which is caused by imperfections in the rubber, etc. One cause may also be caused by a combination of other causes. Thus the puncture may be caused by a rough road surface and by sudden braking, as well as a weak tire, as illustrated.

Fig. 1. Chains of causes

The Cause-Effect Diagram uses a specific layout to display the hierarchy of causes, as illustrated. The angled lines enable more detail to be added than lines at right angles to one another, especially in an informal situation where causes are being added 'on the fly'. Each line indicates either a named cause or a cause area which contributes to the cause line to which it is attached. A cause area is not a cause, but may contain causes. For example, a tire may be a cause area but may not be a cause of an accident. A smooth or punctured tire can be a cause. Cause areas tend to be nouns, whilst causes tend to use verbs.

When determining causes of a problem, the important causes that need to be addressed are seldom all known, let alone the effects of individual causes on the problem and on one another. The Cause-Effect Diagram is often used to address this by acting as an organizing structure within a Brainstorming session, in which case the causes on the final diagram may be a combination of known, suspected and other possible causes.

The Cause-Effect Diagram is often the result of divergent thinking about causes, and must be followed by convergence into the key causes which are to be addressed by further action. To prevent ineffective solutions, these need to be verified as being actual causes before finding solutions for them.

Root causes are those at the ends of chains of causes, and which consequently do not have any sub-causes. Root causes of key causes are often worth addressing.

Fig. 2. The 'bones' of the Cause-Effect Diagram and finding root causes

A lopsided diagram can indicate an over-focus in one area, a lack of knowledge in other areas, or it can simply indicate that the causes are focused in the denser area. A sparse diagram may indicate a lack of general understanding of the problem or just a problem with few possible causes.

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