Cause and Effect (Fishbone) Diagrams

Also known as: Cause and Effect diagram, Ishikawa diagram 

When can we use the Fishbone diagram?

  • When trying to identify possible causes of why something happened or might happen in an orderly manner 
  • Most effective when used in a team or group settings 

Why do we use the Fishbone diagram?

  • Determine root causes.
  • Helps to show causal relationships between contributing factors.
  • Helps to organise potential causes into smaller categories.
  • The prime purpose is to brainstorm all the possibilities that could cause the problem and then drill down to the factor(s) that are causing this issue. It enables teams to focus on why the problem occurs, then once found, eliminate them. 

How do we use the Fishbone diagram?

  1. Agree on the problem statement that is clear and specific. 
  2. Write this on the centre right of the chart - draw a box around it and a horizontal line running to the left of the chart ‘the spine’. 
  3. Now decide on categories - it is easier with new teams to use generic cause categories:
    Methods, Materials, Measurement, Machines, Manpower, and Environment.
    Alternatively you can brainstorm categories in relation to a specific problem.
  4. Draw ‘fish bones’ emanating from the spine; write one category at the top end of one fishbone and draw a box around each one.
  5. Now brainstorm what all the possible causes of the problem are and add these ideas to the most appropriate category – note these can be written into several categories if they relate.
  6. Now consider why these occur? These responses cascade off the cause bones in pen or with post-it notes flowing towards the effect. 
  7. To generate depth of understanding, continue to ask 'why' regarding the identified causes - layering of the bones shows thorough analysis into the causes of the problem.
  8. Once all ideas are exhausted, the focus now turns to the spaces on the chart that generated fewer ideas.
  9. Once completed, add up all the tallies for each cause and select the three with the highest scores. These three issues will now form the basis of additional investigation in order to find the root causes. 
  10. Close the session off with a summary of the event and actions - who is doing what, by when? This helps ensure the team remains focused on the project. 
  11. Finally have regular reviews to check the status of the action plan for implementing change. 

Creating a Fishbone diagram  

  • Materials needed: whiteboard or flipchart, multiple different coloured pens or post it notes.
  • Alternatively start using the Cause and Effect diagram

Resources

References:

https://asq.org/quality-resources/fishbone

https://asq.org/quality-resources/fishbone

http://www.ihi.org/resources/Pages/Tools/CauseandEffectDiagram.aspx