Cause & Effect Diagrams

Dal blog di Learnsigma un articolo che spiega come si costruiscono e a cosa servono i diagrammi causa-effetto di Ishikawa…

AKA Fishbone Diagrams or Ishikawa Diagrams

Kaoru Ishikawa pioneered the cause and effect diagram to explore all the potential causes that result in a single effect.

Description: The fishbone diagram identifies many possible causes for an effect or problem. It can be used to structure a brainstorming session. It immediately sorts ideas into useful categories.

Procedure

What is the problem?

Write the problem in the form of a question in a box on the right hand side of a large sheet of paper. This gives you space to brainstorm causes along a horizontal line with the “bones” labelled:

The 6 M’s
Machine, Method, Materials, Measurement, Man and Mother Nature (Environment) (recommended for manufacturing industry).

The 8 P’s
Price, Promotion, People, Processes, Place / Plant, Policies, Procedures & Product (or Service) (recommended for administration and service industry).

The 4 S’s
Surroundings, Suppliers, Systems, Skills (recommended for service industry).

So for example:

1. MAN – Does the operator have the proper training, experience, and ability to perform the function?
2. METHOD – Are the work instructions available and up-to-date? Do they reflect the best method to perform the task? Are the proper tools available? Are the process parameters specified clearly?
3. MACHINE – Does the machine have the capability to produce the product as specified? Does the machine have the ability to produce the product on a consistent basis? Are there regular routine maintenance and preventative maintenance tasks? Are they performed according to schedule?
4. MATERIAL – Are the correct materials available for the process? What is the quality of the material used in the process? Is there more than one supplier and does quality vary with different suppliers? What types of material problems could exist?
5. MEASUREMENT – Are the measurement instruments adequate for the process? Are they maintained correctly and regularly calibrated? Are the measurement instruments affected by environmental conditions such as temperature, vibration, dirt, etc.?
6. ENVIRONMENT – Is the manufacturing environment affected by temperature, humidity, dust and dirt, power fluctuations or seasonal differences?

Analyse your diagram:

The diagram will shoe all the possible causes of your problem. 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.

To successfully build a cause and effect diagram:

1. Be sure everyone agrees on the effect or problem statement before beginning.
2. Be succinct.
3. For each node, think what could be its causes. Add them to the tree.
4. Pursue each line of causality back to its root cause.
5. Consider grafting relatively empty branches onto others.
6. Consider splitting up overcrowded branches.
7. Consider which root causes are most likely to merit further investigation.

Also

1. It is helpful to pull many people into the construction of these diagrams, as this ensures enough diversity of thought to make sure you get the righ potential root causes.
2. Keep asking “why” until you arrive at something atomic and actionable.
3. The purpose of this tool is to answer a question, then brainstorm about how to fix the identified root cause.
4. Getting more people involved will give them a sense of ownership — and that sense of ownership is very important because now that they feel part of the process, resistance to change will likely be less of a problem.