The Fishbone Diagram

, also called the Ishikawa Diagram, was developed by quality management pioneer Kaoru Ishikawa.  It is also sometimes referred to as the Cause-&-Effect Diagram — because that’s what it focuses on. 

This is a tool used in brainstorming sessions during a Lean Six Sigma project.  It can be used in either the Analyze or Improve phase of the DMAIC problem-solving framework. 

 Why is it called the “Fishbone” Diagram?  Well, take a look.  Could it be called anything else? 

 The parts of the fishbone diagram are:

• The head of the fish contains the Effect, or Outcome, of a process.
• Horizontal branches contain Causes.  (Note the arrows, which indicate the causal relationship.) 
• These are usually divided into 4 – 6 standard categories, depending on the type of business and process under study. 
  • For Manufacturing, a common list of categories is:  People, Materials, Methods, and Machinery / Equipment.
  • For Service, the list might look like:  People, Policies, Procedures, and Machinery / Equipment.  
  • Personally, I like to add the categories of Environment and Measurement as well, bringing the total count up to 6.  
  • The fact is, you should not feel bound by any particular list of categories.  Use what works for your company.
• Sub-branches contain contributing reasons for each Cause.

What to put on the branches?  Well, here’s where brainstorming comes in.  To help guide your team’s brainstorming efforts, you can use the “5 Why’s” approach.  With the 5 Why’s, you keep on asking “Why” until you either identify the root cause, or run screaming out of the room.  (Just kidding.)  Usually, it takes only 5 Why’s — or fewer — to get to the root cause of a particular problem.

 If you have (or ever had) small children, you are familiar with this approach; you just didn’t know that’s what it was. 

“Mommy, why do I have to wear my seatbelt?”  (The first Why.)
“Because that’s the rule.”
“Why is it the rule?” (The second Why.)
“Because I want you to be safe.”
“But why do I need to be safe?”  (The third Why.)
“Because I don’t want you to be hurt if we ever have a car accident.”
“Why don’t you want me to be hurt?” (The fourth Why.)
“Because I love you!”  (Ah-hah!  Root cause, and we didn’t even get to the 5^th Why.) (Note, also, the desire at this point to run screaming from the room.)

Now, having gone through that process for every possible factor contributing to the presence of defects, we can map them onto the fishbone diagram.  This provides an excellent visual aid to avoid leaping to premature conclusions, and to make sure no key factors are missed.

The Fishbone diagram may seem simple, but putting it into practice can be harder than you might think. 

• It’s important to get the right team members / stakeholders into the brainstorming session. 
• It’s also important to manage the group dynamics, so that by the end of the process, all team members have taken ownership of the entire diagram.  You don’t want people remembering which idea was whose. 
• You may wish to break up the brainstorming activity into more than one session with a break in between.  The break can enable some good ideas or missed factors to bubble up to the surface of participants’ minds, which can help the later sub-sessions be more productive. 

In short, the Fishbone diagram can be a useful process improvement tool, helping teams to look beyond the obvious answers to the root causes of defects.

Welcome back to Lean Six Sigma Source! Thanks for your continued support.

Related posts:

1. FISHBONE DIAGRAM: How To Construct A Fishbone Diagram

Tags: brainstorm, cause and effect diagram, defects, dmaic, fishbone diagram, ishikawa diagram, Lean Six Sigma, root cause