The fishbone diagram is the most widely used cause-and-effect analysis tool in Six Sigma. Cause and effect analysis is one of the key tasks in any Six Sigma DMAIC project because half the game is won when finding the correct root causes of a problem (Y).
The team using the fishbone diagram tool should perform the steps listed below.
Agree on the problem statement (also referred to as the impact). This is written on the mouth of the “fish”. Be as clear and specific as possible about the problem. Beware of defining the problem in terms of a solution (we need more of something).
Agree on major categories of problem causes (written as branches of the main arrow). Major categories often include: equipment or supply factors, environmental factors, rules/policy/procedure factors, and people/personnel factors.
Think of all possible causes of the problem. Ask “Why is this happening?” When introducing each idea, the facilitator writes the causal factor as a branch of the appropriate category (puts it in a fishbone diagram). Reasons can be written in several places if they relate to several categories.
He asks again “Why is this happening?” about every cause. Write down the sub-causes of the cause branches.
He continues to ask, “Why?” and generate deeper levels of causes and continue to organize them under related causes or categories. This will help you identify the root causes and then treat them to prevent future problems