What is a cause-and-effect diagram and how is it used in Six Sigma?
The Cause and Effect Diagram is also commonly known as a fishbone because of its fishbone appearance.The box and lines can be thought of as the head and spine of the fish.The box represents the effect and the lines are the causes.
One of the most difficult problems when it comes to implementing Six Sigma is that it is difficult to determine which processes are the most problematic.This is especially true when you have a team of problem solvers all trying to decide where the focus should be.Using a Cause and Effect diagram is one great way to encourage brainstorming among team members. Cause and Effect analysis helps you to think through causes of a problem thoroughly. Their major benefit is that they push you to consider all possible causes of the problem and not just the ones that are most obvious.
The approach combines brainstorming with use of a type of Concept Map.The fishbone diagram will help to visually display the many potential causes for a specific problem or effect.The following steps can be used to solve a problem implementing a Cause and Effect diagram:
1. Identify the problem: Start with stating the problem in the form of a question.This will help in brainstorming since each root cause idea should answer the question.Identify what the problem is, who is involved, and when and where it occurs. Write down everything in detail.Write the problem in a box on the left hand side of a large sheet of paper. This will be the fish ‘head'.Draw a line across the paper horizontally from the box. This gives you space to develop ideas as you brainstorm.
2. Work out the major factors involved: Next, identify the factors that may contribute to the problem. Draw diagonal or vertical lines (bones) coming off of the horizontal line (spine) for each factor and label it.Try to draw out as many possible factors as possible. This is where you need to brainstorm as a group.Although you should modify the categories according to your project and subject matter, there are some standard categories in types of process.These are:
- Service Industries (4P's): Policies, Procedures, People, Plant/Technologies
- Manufacturing Industries (6M's): Machines, Methods, Materials, Measurements, Mother Nature (environment), Manpower (people)
- Process Steps (for example): Determine Customers, Advertise Product, Purchase, Sell Product, Ship Product, Provide Upgrade
3. Identify possible causes: For each of the factors you considered in stage two, 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. For each cause identified, continue to ask 'why does that happen?' and attach that information as another bone of the category branch. This will help get you to the true root of a problem.When you come across a cause that is large or complex, break 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. You should have your team prioritize the key causes identified on the fishbone.If necessary, you may also want to validate these prioritized causes with a larger audience.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.
Cause and Effect analysis (or fishbone analysis) provide a structured way to help you think through all possible causes of a problem.This is a great alternative for problem solving when there is little quantitative data available for analysis.