Cause & effect diagrams


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Cause & effect diagrams

  1. 1. Cause-and-Effect Diagrams (Fishbone Diagrams)It is difficult if not impossible to solve complicated problems without considering many factors and thecause-and-effect relationships between those factors. Defining and displaying those relationshipshelps. The first such cause-and-effect diagram was used by Kaoru Ishikawa in 1943 to explain to agroup of engineers at the Kawasaki Steel Works how various work factors could be sorted andrelated. In recognition of this, these diagrams sometimes are called Ishikawa diagrams. They are alsocalled fishbone diagrams, because they look something like fish skeletons.What can it do for you?Quality problems are typically not simple. They often involve the complex interaction of severalcauses. A cause-and-effect diagram will help you:  Define and display the major causes, sub-causes and root causes that influence a process or a characteristic.  Provide a focus for discussion and consensus.  Visualize the possible relationships between causes which may be creating problems or defects.Cause-and-effect diagrams are particularly useful in the measure and improve phases of Lean SixSigma methodology.How do you do it? 1. Decide which quality characteristic, outcome or effect you want to examine. You might consider Pareto analysis to help you focus on the most important issue. 2. Write your chosen effect on the right side of a paper, board or flipchart and draw a box around it. If you think of this as a fishbone diagram, this is the fish’s head. 3. Draw a straight line to the left, the fish’s backbone. 4. For each primary cause or category of causes, draw a diagonal line slanting from left to the centerline. Alternate these ribs on the top and bottom of the backbone. Label the end of each rib and draw a box around the label. 5. Draw a horizontal line intersecting the appropriate diagonal line and label it to describe each secondary cause that influences a primary cause. Alternate these medium sized bones to the left and right of each rib. 6. In a similar way, draw and label diagonal lines for third level or root causes, small bones, intersecting the secondary cause lines, medium sized bones. 7. Examine the diagram. If certain causes Big Bone seem to have Small Bone a significant Medium Bone effect on the Backbone Characteristic characteristic you are examining, mark them in a special way. Factors (causes) Characteristic (effect) Cause and Effect Diagram
  2. 2. Variation 1: Cause EnumerationSometime it may be very difficult to determine the primary causes to be included in your diagram. Ifthat is the case, after you have determined the characteristic or effect you are examining, follow thesesteps:  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.  Machine, Manpower, Material, Measurement, Method and Environment are frequently used major causes that can apply to many processes.The advantage of the cause enumeration technique is that you stand a much better chance that allcauses will be listed, especially hidden ones, and your diagram will be a complete and useful picture.The disadvantage is that it may be difficult to relate all the causes clearly to the result, making thediagram hard to draw.Variation 2: Process ClassificationSometimes it is more helpful to look at causes in the sequence in which they occur instead ofconsidering overreaching logical categories. With this approach, the center line or backbone followsthe sequence of the process.  Instead of primary causes as the ribs, show the major process steps from left to right.  Construct your cause-and-effect diagram as before.The advantage of this technique is that, since it follows the sequence of the process, it will be easy foreveryone to understand. The disadvantages are that similar causes will appear again and again, andcauses due to a combination of factors will be difficult to show.
  3. 3. Hints for Making Good Diagrams 1. Get input from many people involved in the process. Not only will this make for a more accurate diagram, everyone taking part will gain new knowledge. 2. Make one cause-and-effect diagram for each Critical-To-Quality (CTQ) characteristic you are considering. Trying to include all CTQs on one diagram will make it too large and complicated to be of much use as a problem solving tool. 3. Avoid generalities. Express each cause as concretely as possible. 4. Since you will use your diagram to direct the examination of specific cause-and-effect relationships with data, the characteristic you are considering and all the causal factors should be measurable. If they are not, try to make them measurable or find substitutes. 5. The objective of the cause-and-effect diagram is action. Be sure your causes are broken down to the level at which they can be acted on.Now what?  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 tobe checked, modified or eliminated.
  4. 4. Steven Bonacorsi is the President of the International Standard for Lean Six Sigma(ISLSS) and Certified Lean Six Sigma Master Black Belt instructor and coach. StevenBonacorsi has trained hundreds of Master Black Belts, Black Belts, Green Belts, andProject Sponsors and Executive Leaders in Lean Six Sigma DMAIC and Design forLean Six Sigma process improvement methodologies. Author for the Process Excellence Network (PEX Network / IQPC). FREE Lean Six Sigma and BPM contentInternational Standard for Lean Six SigmaSteven Bonacorsi, President and Lean Six Sigma Master Black Belt47 Seasons Lane, Londonderry, NH 03053, USAPhone: + (1) 603-401-7047Steven Bonacorsi e-mailSteven Bonacorsi LinkedInSteven Bonacorsi TwitterLean Six Sigma Group