By Issa Bass
 

The cause-and-effect diagram also known as fishbone (because of its shape) or Ishikawa diagram (after its creator) is used to synthetize the different causes of an outcome. It is an analytical tool that provides a visual and systematic way of linking different causes (input) to an effect (output). It can be used in the design phase of a production process as well as in an attempt to identify the root causes of a problem. The effect is considered positive when it is an objective to be reached as in the case of a manufacturing design, it is negative when it addresses a problem being investigated

The building of the diagram is based on the sequence of events, "sub-causes" are classified according to how they generate "sub-effects" and those "sub-effects" become the causes of the outcome being addressed.

The Fishbone diagram does help visually identify the root causes of an outcome but it does not quantify the level of correlation between the different causes and the outcome. Further statistical analysis is needed to determine which factors contribute the most to creating the effect. The Pareto Analysis is a good tool for that purpose but it still pre-requires data gathering. Regression analysis allows the quantification and the determination of the level of association between causes and effects. A combination of Pareto and Regression Analysis can help not only determine the level of correlation but also stratify the root causes. The causes are stratified hierarchically according to their level of importance and their areas of occurrence

The first step in constructing a Fishbone diagram is to clearly define the effect being analyzed.

The second step will consist into gathering all the data about the Key Process Input Variables (the KPIV), the potential causes (in the case of a problem) or requirements (in the case of the design of a production process) that can affect the outcome.

The third step will consist in categorizing the causes or requirements according to their level of importance or areas of pertinence. The most frequently used categories are:

• Manpower, Machine, Method, Measurement and Materials  for manufacturing
• Equipment, Policy, Procedure, Plant and  People for services

Sub-categories are also classified accordingly; different kinds of machines and computers for instance can be classified as sub categories of equipment.

The last step is the actual drawing of the diagram.

The following diagram is an example of a Cause-and-Effect diagram that explains why a production plant is producing an excessive amount of defects.

About the author
Issa Bass is the managing editor of SixSigmaFirst. He can be reached at issa@sixsigmafirst.com

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