Cause and Effect Diagram in Quality Control

The cause-and-effect diagram is one of Total Quality Management tools. It is also called an Ishikawa diagram because Kaoru Ishikawa developed it or fishbone diagram because the complete diagram resembles a fish skeleton. The diagram illustrates the main causes and subcases leading to an effect. It is used to organize and graphically display multiple causes with a particular effect.

It is tool for team brainstorming used to identify potential root causes to problems. Because of its function it may be referred to as a cause and-effect diagram. In a typical Fishbone diagram, the effect is usually a problem needs to be resolved, and is placed at the "fish head". The causes of the effect are then laid out along the "bones", and categorized into different types along the branches. Further causes can be laid out alongside further side branches.  

This tool can be applied at different stages of construction projects to analyze causes of rejection and take requisite preventive or remedial action; develop a system for the preparation and processing of design drawings and documents, execution/installation of work, processing of shop drawings; and many other applications.

Cause and Effect Diagram Objectives

The leading goal of the cause and effect diagram is to illustrate in a diagrammatic way the relationship between a given consequence and all factors that effect this consequence. The main objectives of this tool are:

•                Determining the root causes of a problem.
•        Focusing on a specific issue without resorting to irrelevant discussion.
•        Identifying areas where there is a lack of data.

The Importance of Cause and Effect Diagram

•             Encourages creativity through a brainstorming process.
•      Useful in brainstorming sessions to focus conversation.
•      Provides a visual image of the problem and potential categories of causes.
•     Allows various categories of causes to be explored and uncovers bottlenecks in your processes.

What is Cause and Effect Diagram?

Dr. KaoruIshikawa, a Japanese quality control statistician, invented the fishbone diagram. It is often also referred to as the Ishikawa diagram. The fishbone diagram is an analysis tool that provides a systematic way of looking at effects and the causes that create or contribute to those effects. Because of the function of the fishbone diagram, it may be referred to as a cause-and-effect diagram. The design of the diagram looks much like the skeleton of a fish. Therefore, it is often referred to as the fishbone diagram. A cause-and-effect diagram can help identify the reasons why a process goes out of control. Often the fishbone diagram can be used to summarize the results of a brainstorming session, identifying the causes of a specified undesirable outcome. It helps to identify root causes and ensures a common understanding of the causes.

There are six steps that are used to perform a cause-and-effect analysis:

Step 1. Identify the problem to analyze its technical cause.

Step 2. Select an interdisciplinary brainstorm team.

Step 3. Draw a problem box and prime arrows.

Step 4. Specify major categories contributing to the problem.

Step 5. Identify a defect cause.

Step 6. Identify corrective action and perform the analysis in the same manner as for the cause and effect analysis.

The cause and effect diagram can be used:

• When identifying possible causes of a problem 
• When the team’s thinking tends to fall into ruts

Procedure to Create Cause and Effect Diagram

•          Agree on a problem statement (effect). Write it at the center right of the flipchart. Draw a box around it and draw a horizontal arrow running to it.
•          Brainstorm the major categories of causes of the problem. If there is difficulty here, use generic headings: method, machines (equipment), people (manpower), materials, measurement, and environment. Write the categories of causes as branches from the main arrow.
•          Brainstorm all the possible causes of the problem. Ask “Why does this happen?” As each idea is given, the facilitator writes it as a branch from the appropriate category. Causes can be written in several places if they relate to several categories or multiple relationships.
•    Ask again, “Why does this happen?” about each cause. Write sub causes branching off the causes. Continue to ask “Why” and generate deeper levels of causes. Layers of branches indicate causal relationships.
•     When the team runs out of ideas, focus attention on places on the fishbone where ideas are few.

Benefits of Cause and Effect Diagram

• Helps determine root causes.
• Encourages group participation.
• Uses an orderly, easy-to-read format to diagram cause and effect relationships.
• Indicates possible causes of variation.
• Increases knowledge of the process by helping everyone to learn more about the factors at work and how they relate.
• Identifies areas for collecting data.

Good and bad Cause and Effect Diagram

A good cause and effect diagram is one which explores all possibilities so it is likely to be large and complex-looking sprouts for each new related idea noted down. Be suspicious of cause and effect diagrams with few factors, or which are neat and well ordered. These may reflect a lack of knowledge of the situation, or show that the effort to draw the diagram was not creative and exhaustive enough.

The cause and effect diagram can also be drawn with right angles, which makes it less tangled, and easier to see what layer of causality is being considered at any given time.

Tips to Make a Useful Cause and Effect Diagram

•             Use an automatic diagram software instead of hand drawing to save time and effort.
•        See some examples before drawing. Understanding fishbone diagram is best achieved by studying examples.
•     Learn from others. Seek their help. Involve as many related people as possible. Collect their ideas and advice.
•      Brainstorm potential causes by asking why questions. This helps to reveal causes that are not immediately obvious.
•        Classify the causes into different categories logically so that you can present them in a more structured way.
•       Leave enough space between primary causes since you may need to add many secondary, thirdly level or even more causes.

Advantages and Disadvantages

Advantages

•          Fishbone diagrams permit a thoughtful analysis that avoids overlooking any possible root causes for a need.
•    The fishbone technique is easy to implement and creates an easy to‐understand visual representation of the causes, categories of causes, and the need.
•      By using a fishbone diagram, you are able to focus the group on the ʺbig pictureʺ as to possible causes or factors influencing the problem/need.
•        Even after the need has been addressed, the fishbone diagram shows areas of weakness that ‐ once exposed ‐ can be rectified before causing more sustained difficulties.

Disadvantages

•      The simplicity of a fishbone diagram can be both its strength and its weakness. As a weakness, the simplicity of the fishbone diagram may make it difficult to represent the truly interrelated nature of problems and causes in some very complex situations.
•          Unless you have an extremely large space on which to draw and develop the fishbone diagram, you may find that you are not able to explore the cause and effect relationships in as much detail as you would like to.