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Causes and Effect Diagrams in Quality Management

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CAUSE EFFECT DIAGRM IN INDUSTRY

Definition of Cause & Effect Diagram

The cause & effect diagram is the brainchild of Kaoru Ishikawa, who pioneered quality management processes in the Kawasaki shipyards, and in the process became one of the founding fathers of modern management. The cause and effect diagram is used to explore all the potential or real causes (or inputs) that result in a single effect (or output). Causes are arranged according to their level of importance or detail, resulting in a depiction of relationships and hierarchy of events. This can help you search for root causes, identify areas where there may be problems, and compare the relative importance of different causes.

Causes in a cause & effect diagram are frequently arranged into four major categories. While these categories can be anything, you will often see:

  • manpower, methods, materials, and machinery (recommended for manufacturing)
  • equipment, policies, procedures, and people (recommended for administration and service).

These guidelines can be helpful but should not be used if they limit the diagram or are inappropriate. The categories you use should suit your needs. At SkyMark, we often create the branches of the cause and effect tree from the titles of the affinity sets in a preceding affinity diagram.

The C&E diagram is also known as the fishbone diagram because it was drawn to resemble the skeleton of a fish, with the main causal categories drawn as "bones" attached to the spine of the fish, as shown below.

The fishbone diagram, as originally drawn by Kaoru Ishikawa, is the classic way of displaying root causes of an observed effect

Cause & effect diagrams can also be drawn as tree diagrams, resembling a tree turned on its side. From a single outcome or trunk, branches extend that represent major categories of inputs or causes that create that single outcome. These large branches then lead to smaller and smaller branches of causes all the way down to twigs at the ends. The tree structure has an advantage over the fishbone-style diagram. As a fishbone diagram becomes more and more complex, it becomes difficult to find and compare items that are the same distance from the effect because they are dispersed over the diagram. With the tree structure, all items on the same causal level are aligned vertically.

History

Ishikawa diagram, in fishbone shape, showing factors of Equipment, Process, People, Materials, Environment and Management, all affecting the overall problem. Smaller arrows connect the sub-causes to major causes.

Ishikawa diagrams were proposed by Kaoru Ishikawa in the 1960s, who pioneered quality management processes in the Kawasaki shipyards, and in the process became one of the founding fathers of modern management.

It was first used in the 1960s, and is considered one of the seven basic tools of quality control. It is known as a fishbone diagram because of its shape, similar to the side view of a fish skeleton.

Mazda Motors famously used an Ishikawa diagram in the development of the Miata sports car, where the required result was "Jinba Ittai" or "Horse and Rider as One". The main causes included such aspects as "touch" and "braking" with the lesser causes including highly granular factors such as "50/50 weight distribution" and "able to rest elbow on top of driver's door". Every factor identified in the diagram was included in the final design. The Cause & Effect (CE) diagram, also sometimes called the ‘fishbone' diagram, is a tool for discovering all the possible causes for a particular effect. The effect being examined is normally some troublesome aspect of product or service quality, such as 'a machined part not to specification', 'delivery times varying too widely', 'excessive number of bugs in software under development', and so on, but the effect may also relate to internal processes such as 'high rate of team failures'.

The major purpose of the CE Diagram is to act as a first step in problem solving by generating a comprehensive list of possible causes. It can lead to immediate identification of major causes and point to the potential remedial actions or, failing this, it may indicate the best potential areas for further exploration and analysis. At a minimum, preparing a CE Diagram will lead to greater The CE Diagram was invented by Professor Kaoru Ishikawa of Tokyo University, a highly regarded Japanese expert in quality management. He first used it in 1943 to help explain to a group of engineers at Kawasaki Steel Works how a complex set of factors could be related to help understand a problem. CE Diagrams have since become a standard tool of analysis in Japan and in the West in conjunction with other analytical and problem-solving tools and techniques.

CE Diagrams are also often called Ishikawa Diagrams, after their inventor, or Fishbone Diagrams because the diagram itself can look like the skeleton of a fish.

Typical categories are:

The 4 M's (used in manufacturing) understanding of the problem.

  • Machine (Technology)
  • Method (Process/Inspection)
  • Material (Raw, Consumables etc.)
  • Man Power (physical work)/Mind Power (Brain Work): Kaizens, Suggestions

The 8 P's (used in service industry)

  • Product=Service
  • Price
  • Place
  • Promotion
  • People
  • Process
  • Physical Evidence
  • Productivity & Quality

The 4 S's (used in service industry)

  • Surroundings
  • Suppliers
  • Systems
  • Skills

More M's

  • Mother Nature (Environment)
  • Measurement (Inspection)
  • Maintenance
  • Money Power
  • Management

Why Use a Cause & Effect Diagram?

A cause & effect diagram helps to determine the causes of a problem or quality characteristic using a structured approach. It encourages group participation and utilizes team knowledge of the process. It uses an orderly, easy-to-read format to diagram cause-and-effect relationships. It increases knowledge of the process by helping everyone to learn more about the factors at work and how they relate. It indicates possible causes of variation in a process and identifies areas where data should be collected for further study.

Example of cause effect diagram

How to draw CE diagram in industries

This is a three step process.

Step 1

Write down the effect to be investigated and draw the 'backbone' arrow to it. In the example shown below the effect is 'Incorrect deliveries'.

Step 2

Identify all the broad areas of enquiry in which the causes of the effect being investigated may lie. For incorrect deliveries the diagram may then become:

For manufacturing processes, the broad areas of enquiry which are most often used are Materials (raw materials), Equipment (machines and tools), Workers (methods of work), and Inspection (measuring method).

Step 3

This step requires the greatest amount of work and imagination because it requires you (or you and your team) to write in all the detailed possible causes in each of the broad areas of enquiry. Each cause identified should be fully explored for further more specific causes which, in turn, contribute to them.

You continue this process of branching off into more and more directions until every possible cause has been identified. The final result will represent a sort of a 'mind dump' of all the factors relating to the effect being explored and the relationships between them.

Different types of CE Diagram

There are three different types of CE Diagram. The basic type explained above is called the Dispersion analysis type. The other two are the Production process classification type and the Cause enumeration type.

Production classification type

This type differs from the basic type above in that each discrete stage in the production process leading up to the effect being examined is shown along the main arrow or 'backbone' of the diagram. Possible causes are then shown as branches off these as shown in the illustration overleaf.

This type of CE Diagram is often easier to construct and understand because those involved are already familiar with each of the production steps identified.

Cause enumeration type

This is not so much a different type of diagram but a different method of constructing a diagram. Instead of building up a chart gradually (starting with the 'backbone', deciding broad areas, then adding more and more branches), you postpone drawing the chart and simply list all the possible causes first. Then draw the chart in order to relate the causes to each other. This method has the advantage that the list of possible causes will be more comprehensive because the process has a more free-form nature. The disadvantage is that it is more difficult to draw the diagram from this list rather than from scratch.

This method of drawing a CE Diagram can be used in conjunction with Brainstorming by using it to distil the brainstorm output down into a logical and useable set of information.

Good and bad CE diagrams

A good CE diagram is one which explores all possibilities so it is likely to be large and complex-looking as twig after twig sprouts for each new related idea noted down. Be suspicious of CE 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.

How to Use the Tool in cause and effect diagram :

Follow these steps to solve a problem with a Cause and Effect Diagram:

1. Identify the problem:

Write down the exact problem you face in detail. Where appropriate identify who is involved, what the problem is, and when and where it occurs. Write the problem in a box on the left hand side of a large sheet of paper. Draw a line across the paper horizontally from the box. This arrangement, looking like the head and spine of a fish, gives you space to develop ideas.

2. Work out the major factors involved:

Next identify the factors that may contribute to the problem. Draw lines off the spine for each factor, and label it. These may be people involved with the problem, systems, equipment, materials, external forces, etc. Try to draw out as many possible factors as possible. If you are trying to solve the problem as part of a group, then this may be a good time for some brainstorming.

Using the 'Fish bone' analogy, the factors you find can be thought of as the bones of the fish.

3. Identify possible causes:

For each of the factors you considered in stage 2, brainstorm possible causes of the problem that may be related to the factor. Show these as smaller lines coming off the 'bones' of the fish. Where a cause is large or complex, then it may be best to break the it down into sub-causes. Show these as lines coming off each cause line.

4. Analyze your diagram:

By this stage you should have a diagram showing all the possible causes of your problem that you can think of. Depending on the complexity and importance of the problem, you can now investigate the most likely causes further. This may involve setting up investigations, carrying out surveys, etc. These will be designed to test whether your assessments are correct.

There are three main applications of cause-and-effect diagrams in industries:

1. Cause enumeration is one of the most widely used graphical techniques for quality control and improvement. Sometime it may be very difficult to determine the primary causes to be included in the diagram. If that is the case, after we have determined the characteristic or effect we are examining, we follow these steps:
• Use brainstorming to create a list of all the possible causes. The list will contain a mixture of primary, secondary and tertiary (or big bone, middle sized bone and small bone) causes.
• Sort the list by grouping causes that are related.
• Identify or name each major grouping and make your cause-and-effect diagram. (Thus cause enumeration facilitates the identification of root causes because all conceivable causes are listed.)
• Machine, Manpower, Material, Measurement, Method and Environment are frequently used major causes that can apply to many processes.

2. In Dispersion analysis, each major cause is thoroughly analyzed by investigating the sub-causes and their impact on the quality characteristics (or effect) in question. The key to this diagram's effectiveness lies in the reiteration of the question, "Why does this dispersion (cause) occur?" This diagram helps us outlining the reasons for any variability, or dispersion. Unlike cause enumeration where smaller causes that are considered insignificant are still listed, in dispersion analysis, causes that don't fit the selected categories are not listed. In other words, sometimes small causes are not isolated or observed. Consequently, it is possible that some root causes will not be identified in dispersion analysis.

3. When cause-and-effect diagrams are constructed for process analysis, the emphasis is on listing the causes in the sequence in which the operations are actually conducted. The advantage of this diagram is that, since it follows the sequence of the production process, it is easy to assemble and understand. The disadvantage is that similar causes appear again and again, and causes due to a combination of more than one factor are difficult to illustrate.

Fishbone diagram

Uses of cause effect diagram in industries:-

  • Use your diagram to develop a common understanding of the factors potentially influencing or causing a quality problem.
  • Use your diagram as a road map for collecting data to verify the causal relationship of various factors to the characteristic.
  • Continue to annotate and modify your diagram as you verify relationships and learn more.

Using a cause-and-effect diagram this way will help you to see which factors in your process need to be checked, modified or eliminated

Example The example below shows a Cause & Effect diagram drawn by a manager who is having trouble getting cooperation from a branch office

If the manager had not thought the problem through, he might have dealt with the problem by assuming that people were being difficult.

Instead he might think that the best approach is to arrange a meeting with the Branch Manager. This would allow him to brief the manager fully, and talk through any problems that he may be facing.

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.

 


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