Fishbone Diagram or the Ishikawa Diagram is criticized for an inability to distinguish between necessary and sufficient conditions. The Ishikawa diagram merely shows causes of an event. 

 

Necessary conditions  - A set of circumstance in absence of which, the event cannot occur.

Sufficient set of conditions - A set of conditions which if present will ensure that the event will occur. 

 

Just to distinguish these, here is an example. Writing an exam is a necessary condition to pass the exam but is not a sufficient condition. If you are drawing a fishbone diagram for "only a few people pass an exam" - you may mention "many people do not write the exam" as one reason. This is not a sufficient condition - even if more people had written the exam, it may not have ensured that more would have cleared. In many cases, people who do not write are the people who lack confidence or knowledge. 

 

Well, this opens a room for debate. How would the necessary/sufficient distinction make our root cause analysis more useful? Please explain with an example.

1. In class room example of Renu and Deepak, If I want to find out when Renu winks? 'When ever Deepak is present' is a necessary condition but not sufficient because it is not proved that every time she winks in presence of Deepak (because of Pitaji factor). Sufficient condition is "Presence of Deepak" and "Absence of Renu's father".

 

2. Why all prefer Benchmark Six Sigma for GB and BB?

 

Necessary Condition is :

1. Expert Faculty in Six Sigma 
2. Exemplar Global (Previously RABQSA)

Sufficient Condition is:

1. Placement Support
2. Project Support  

Distinction between necessary and sufficient conditions can be quite helpful in finding the root cause.:-

 

For Example : - 

Necessary :

• Having four sides is necessary for being a square.
• Being rich is not necessary for being happy since a poor person can be happy too.

Sufficient :

• Being a square is sufficient for having four sides.
• Loyalty is not sufficient for honesty because one might have to lie in order to protect the person one is loyal to.