The 5 Whys can help you uncover root causes quickly. However, making a single mistake in any question or answer can produce false or misleading results. You may find that there is more than one root cause for each non‐conformance; corrective actions should be implemented for each of these.
Fishbone Diagram
One of the more popular tools used in root cause analysis is the fishbone diagram, otherwise known as the Ishikawa diagram, named after Kaoru Ishikawa, who developed it in the 1960s. A fishbone diagram is perhaps the easiest tool in the family of cause and effect diagrams that engineers and scientists use in unearthing factors that lead to an undesirable outcome.
A fishbone diagram is a visual way to look at cause and effect. It is a more structured approach than some other tools available for brainstorming causes of a problem (e.g., the Five Whys tool). The problem or effect is displayed at the head or mouth of the fish. Possible contributing causes are listed on the smaller “bones” under various cause categories. A fishbone diagram can be helpful in identifying possible causes for a problem that might not otherwise be considered by directing the team to look at the categories and think of alternative causes. Include team members who have personal knowledge of the processes and systems involved in the problem or event to be investigated.
Fishbone Diagram Structure
The left side of the diagram is where the causes are listed. The causes are broken out into major cause categories. The causes you identify will be placed in the appropriate cause categories as you build the diagram.
The right side of the diagram lists the effect. The effect is written as the problem statement for which you are trying to identify the causes.
The diagram looks like the skeleton of a fish, which is where the fishbone name comes from.
How to Create a Cause and Effect Diagram
A cause and effect diagram can be created in six steps.
1 Draw Problem Statement
2 Draw Major Cause Categories
3 Brainstorm Causes
4 Categorize Causes
5 Determine Deeper Causes
6 Identify Root Causes
1 Draw Problem StatementThe first step of any problem‐solving activity is to define the problem. You want to make sure that you define the problem correctly and that everyone agrees on the problem statement.Once your problem statement is ready, write it in the box on the right‐hand side of the diagram.
2 Draw Major Cause CategoriesAfter the problem statement has been placed on the diagram, draw the major cause categories on the left‐hand side and connect them to the “backbone” of the fishbone chart.In a manufacturing environment, the traditional categories areMachines/EquipmentMethodsMaterialsPeopleIn a service organization, the traditional categories are…PoliciesProceduresPlantPeopleYou can start with those categories or use a different set that is more applicable for your problem. There isn't a perfect set or specified number of categories. Use what makes sense for your problem.Cause and Effect Diagram ‐ Major Cause Categories
3 Brainstorm CausesBrainstorming the causes of the problem is where most of the effort in creating your Ishikawa diagram takes place.Some people prefer to generate a list of causes before the previous steps in order to allow ideas to flow without being constrained by the major cause categories.However, sometimes the major cause categories can be used as catalysts to generate ideas. This is especially helpful when the flow of ideas starts to slow down.
4 Categorize CausesOnce your list of causes has been generated, you can start to place them in the appropriate category on the diagram.Draw a box around each category label and use a diagonal line to form a branch connecting the box to the spine.Write the main categories your team has selected to the left of the effect box, some above the spine and some below it.Ideally, each cause should only be placed in one category. However, some of the “People” causes may belong in multiple categories. For example, Lack of Training may be a legitimate cause for incorrect usage of Machinery as well as ignorance about a specific Method.Establish the major causes, or categories, under which other possible causes will be listed. You should use category labels that make sense for the diagram you are creating.Identify as many causes or factors as possible and attach them as subbranches of the major branchesIshikawa Diagram ‐ Categorize Causes
5 Determine Deeper CausesEach cause on the chart is then analyzed further to determine if there is a more fundamental cause for that aspect. This can be done by asking the question, “Why does it happen?”This step can also be done for the deeper causes that are identified. Generally, you can stop going deeper when a cause is controlled one level of management removed from your group. Use your judgment to decide when to stop.Fishbone Chart ‐ Deeper Causes
6 Identify Root CausesThe final step for creating a fishbone diagram is to identify the root causes of the problem. This can be done in several ways…Look for causes that appear repeatedlySelect using group consensus methodsSelect based on frequency of occurrenceFishbone diagrams are an excellent way to explore and visually depict the causes of a problem. They enable the root causes of a problem to be determined. This will help you be more effective by focusing your actions on the true causes of a problem and not on its symptoms. It Encourages group participation, Uses an orderly, easy‐to‐read format to diagram cause and effect relationships.
Fault Tree Analysis
Fault tree analysis helps determine the root cause of failure of a system using Boolean logic to combine a series of lower level events. FTA is a deductive analysis depicting a visual path of failure. It is a top‐down analysis that helps determine the probability of occurrence for an undesirable event. The analysis creates a visual record showing the logical relationships between events and failures that lead to the undesirable event. It easily presents the results of your analysis and pinpoints weaknesses in the system.
The fault tree analysis (FTA) was first introduced by Bell Laboratories and is one of the most widely used methods in system reliability, maintainability and safety analysis. It is a deductive procedure used to determine the various combinations of hardware and software failures and human errors that could cause undesired events (referred to as top events) at the system level.
To do a comprehensive FTA, follow these steps:
1 Define the fault condition, and write down the top‐level failure.
2 Using technical information and professional judgments, determine the possible reasons for the failure to occur. Remember, these are level two elements because they fall just below the top‐level failure in the tree.
3 Continue to break down each element with additional gates to lower levels. Consider the relationships between the elements to help you decide whether to use an “and” or an “or” logic gate.
4 Finalize and review the complete diagram. The chain can only be terminated in a basic fault: human, hardware, or software.
5 If possible, evaluate the probability of occurrence for each of the lowest
