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Why-Why Diagram

Quality Tools > Tools of the Trade > 24: Why-Why Diagram


One of the most important questions in quality is ‘Why?’. From Kaoru Ishikawa’s early usage of cause-effect diagrams in the early 1940s and Toyota’s use of ‘Five Whys’ in the early 1950s as they developed their famous Toyota Production System, the drive to find root causes of problems has been the secret of many quality success stories.


How it works

Why-Why works by repeatedly asking the same question of a problem, breaking down the cause or solution into more and more explicit elements. At each stage, there can be multiple answers to the ‘Why’ questions, which results in a hierarchical tree-structure.

Making this tree visible gives several advantages:

§       It allows a group of people to share the mental model of the situation and hence work more harmoniously on it.

§       It allows re-examination of parts of the analysis, so you can change, remove or add to it at any time. This supports the non-linear way in which we tend to think.

§       It allows you to consciously not to follow some paths, digging only into the most likely areas.


Why-Why-Why Diagram


How to do it

1. Write the problem on a Post-it Note and place it to the left of a large work area on the wall. A big sheet of brown paper or several flip-chart pages taped together can help.

2. Ask ‘What are the main sub-areas that may lead to this problem?’. Write each of these on a Post-it Note and stick them up, well spaced out (to allow lower-level trees to develop) to the right of the problem Post-it Note.

3. Repeat this sequence of breaking down the problem once more, this time simply asking ‘Why does this happen?’

4. After this third level, a fourth detailed level would overcrowd the work area, so start to focus by only developing likely-looking causes or solution areas. In each of these, keep asking Why until you have a root cause or a complete solution.

5. When you have completed the analysis, discuss and identify the key cause to address. When examining the diagram, the same sub-cause may turn up several times in various places. This may well mean that if these are addressed, then you will kill several birds with one stone, getting multiple benefits from one action.


Next time: Value Analysis


This article first appeared in Quality World, the journal of the Institute for Quality Assurance

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