- Fishbone diagrams are used to identify the key Causes (Xs) that influence the output measure (Y).
- The major categories are used to direct the team in asking “Why?” until actionable inputs of the effect are identified.
- The categories ensure that all inputs are considered, e.g. not just milk (a material) but also its temperature (a measure) and the milk jug (equipment). By just using the process map and brainstorming, some of these inputs might be overlooked.
The Fishbone Diagram has a number of categories to consider and might stimulate ideas about inputs missed during brainstorming. It is a tool that helps the team think of potential root causes to the problem from a number of different perspectives.
The tool encourages teams to look at multiple categories of inputs to discover the root cause(s). It helps reach a common understanding of potential causes and exposes gaps in existing knowledge.
It is critical for the team to agree upon the problem under study. In most cases, the problem will be identical to the definition of Y.
STEP 2: As a team, come to consensus on the 4-5 major cause categories to use on the fishbone diagram. Write the names of the major cause categories above and below the horizontal line. Connect them to the “back bone” of the fish.
Typical cause categories include:
- Machines (equipment)
- Methods/procedures (how work is done/tasks and steps)
- Materials (components or raw materials)
- People (the human element)
- Mother Nature (Environment)
- Measurement (data collection)
- Policies (higher level decision rules)
Brainstorming can be done by category or can be done prior to placing the potential causes on the major bones. It is recommended that the entire list of possible causes be generated first before attempting to place them on the major category bones. This keeps the creative flow of ideas.
STEP 5: For each cause, drill down into “causes of the causes” by asking repeatedly, “why does this happen?” Arrange contributing causes on the smaller bones.