Ishikawa (Fishbone/Cause-and-Effect) diagram
Posted by isoeasy on May 9, 2006
The Ishikawa diagram (also known as a Cause-and-Effect or Fishbone diagram) is a graphical method for finding the most likely causes for an undesired effect. Kaoru Ishikawa, a famous Japanese consultant developed this method in the 1960s.
Because of its shape, it is also known as the fishbone diagram. Another name for this technique is: the cause-and-effect diagram. The fishbone diagram is a method/tool used in a root cause analysis.The Ishikawa diagram is one of the seven basic tools of quality control, which include the histogram, Pareto chart, check sheet, control chart, cause-and-effect diagram, flowchart, and scatter diagram.
How to make the diagram
Step 1: Take a sheet of paper and draw a box on the right side of the paper. Draw a horizontal line from the left side of the box to the right. Write in the box the effect for which you want to find the causes.
Step 2: Starting from the horizontal line, draw four to six short diagonal lines in the direction the left upper and left lower corner of the paper. These are the main bones of the diagram. Label them with categories you know will span the whole problem space. For example, a business may use: management, manpower, machines and materials (the 4 M's).
Step 3: Identify as many causes under each category and add them to the corresponding category. Detail each cause further (recursively) to the lowest level possible.Analyse this diagram to identify the causes that require deeper investigation. As fishbone diagram identify only potential causes, it may be a good idea to use a Pareto Chart to determine the cause(s) to focus on first. The Cause-and-Effect diagram can be used by individuals or teams; probably most effectively by a group. A typical utilization is the drawing of a diagram on a blackboard by a team leader who first presents the main problem and asks for assistance from the group to determine the main causes which are subsequently drawn on the board as the main bones of the diagram. The team assists by making suggestions and, eventually, the entire cause and effect diagram is filled out. Once the entire fishbone is complete, team discussion takes place to decide what are the most likely root causes of the problem. These causes are circled to indicate items that should be acted upon, and the use of the tool is complete.
* Take care to identify causes rather than symptoms.
* Post diagrams to stimulate thinking and get input from other staff.
*Self-adhesive notes can be used to construct Ishikawa diagrams. Sources of variation can be rearranged to reflect appropriate categories with minimal rework.
* Insure that the ideas placed on the Ishikawa diagram are process variables, not special caused, other problems, tampering, etc.
* Review the quick fixes and rephrase them, if possible, so that they are process variables