Manufacturing Breakthrough Blog
Tuesday March 17, 2015
In my last posting, Six Sigma and the Causal Chain, I asked you the question below, so let’s talk about each of the potential answers.
When attempting to solve a problem, the only way to solve it is:
- One at a time, eliminate all of the symptoms (7.69%)
- Change something and see what happens (0.00%)
- Lay out the ordered sequence of events linking the potential causes to predicted effects (89.74%)
- Flip a coin to decide what to try (2.56%)
Choice 1 suggests we eliminate all of the symptoms, one by one. If we select this option, we really won’t solve our problem, but rather will just apply a bandage of sorts. Without finding the cause of the symptoms, eventually the symptoms will return because the source is still there, hiding. Choice 2 is similar in that changing something to see what happens doesn’t get us any closer to the source of the problem. In fact, changing something to see what happens will probably confuse us because we might end up with new symptoms. Choice 4 is obviously not a good solution because we’re just trying things instead of searching for cause and effect relationships.
So that leaves us with Choice 3 whereby we lay out the ordered sequence of events to get us closer to establishing a cause and effect relationship and eventually develop a viable solution. This is the real power of the Causal Chain. The bottom line is that until we establish the true cause and effect relationship of the problem we are trying to solve, we will flounder. Let’s look at another tool that helps us do this…..the Cause & Effect Diagram or, as it is also known, the Fishbone Diagram or the Ishikawa Diagram.
The Cause & Effect Diagram
When investigating production problems, it is imperative that we take the time to analyze the various potential causes responsible for the effect we are seeing. A Fishbone Diagram is a great tool to accomplish this. While this diagram does not necessarily indicate the relative magnitude of each cause, it does help limit the scope of subsequent data collection efforts. A Cause & Effect/Fishbone diagram is best developed in a brainstorming session with employees directly involved in the process.
Each branch off the main problem line represents a potential cause leading to the problem. Additional contributing factors are also shown for each branch. As a side note, these diagrams resemble the bones of a fish, hence the term Fishbone Diagram. So let’s see a graphic of what they look like.
The figure above represents the classic drawing of what a C & E or Fishbone Diagram looks like. At the far right we see the “Problem Definition” which should be worded in such a way that there is no confusion about the problem. In this drawing we also see boxes labeled as “Cause Category.” In a classic C & E Diagram the typical names of these categories are Man, Materials, Methods and Machine (the Four Ms). The potential root causes for problems in the Categories are added in each of the “Potential Contributor” boxes.
The C & E Diagram is fundamentally used as a brainstorming tool that groups possible causes of a problem into broad headings or categories. In other words, when selecting the various categories for your C & E Diagram, make them relate to your own industry or the specific problem you’re trying to solve. You then proceed to brainstorm all of the potential contributors (i.e. potential root causes) until you’re satisfied that you’ve listed the major ones. Let’s look at a simple example of a C & E Diagram.
A Cause & Effect Diagram Example
In the example below, we see that “Bar Transfer Drive Trips” is listed as the problem. The next logical question(s) to ask are “why is it tripping?” or “what caused it to trip?” Instead of the Four Ms listed earlier, the headings selected for the Cause Categories for this problem are People, Design, Maintenance and Environment.
In each of the potential contributor boxes, we insert those things that are potentially causing the problem. Once the list of potential root causes is complete, each one is then individually assessed to determine which are most likely causing the problem so solutions can be developed. It’s important to understand that the Cause & Effect Diagram is useful to determine what could have caused a problem and not how to correct it. Once you’ve completed your assessment, then it’s simply a matter of testing the potential root causes in their natural order of ranking (i.e. highest probability to lowest probability) of problem causes.
In my next posting we will discuss another very useful tool in the Six Sigma tool kit, the Pareto Chart. Like the Causal Chain and the C & E Diagram, the Pareto Chart is a very useful tool for solving problems. However, as you will see, the Pareto Chart helps you identify the order to “attack” potential root causes. As always, if you have any questions or comments about any of my postings, just leave me a message and I will respond.
Until next time.