A cause/effect diagram was first known by the name fishbone diagram because it looks like the skeleton of a fish. It was first made popular by Dr. Ishikawa back in the late 1970s and early 80s.
Generally, the cause/effect diagram is drawn on a large whiteboard or a large flipchart. The effect is written at the 3 o’clock position. A horizontal line divides the whiteboard into two equal parts. Many times the 6M’s (i.e., Man (in the generic sense), Machine, Material, Method, Measurement, and Mother Nature) are used as branches off of the horizontal line that lead to the effect on the right-hand side of the whiteboard.
The idea is to list out as many possible causes, and sub causes, and sub causes of sub causes (the use of five whys is sometimes appropriate) until the team runs out of ideas. As with brainstorming, you want this to be a fast-moving exercise so that the team can solicit as many ideas in a short period of time as possible.
Use: You would use a cause/effect diagram if you have some effect (typically a problem) and you want to list as many causes as possible so that you can begin to understand where the root cause of the problem resides. Think of some effect – for example – the grass won’t grow underneath the shade tree. Grass won’t grow is the effect. There are a number of reasons why the grass isn’t growing. It is these assignable causes that you want to understand. So, you gather together a small team of subject matter experts and you brainstorm as many possible reasons why the grass is not growing as possible. Through the use of a cause/effect diagram, the team might come up with 100 reasons why they think the grass isn’t growing. Now, once the team has identified these hundred possible reasons, now the team is going to boil down those hundred reasons into five or six main reasons why the grass isn’t growing. The team will then use other tools of six sigma to understand whether the grass not growing is due to some main effect reason (e.g., soil acidity) or a combination of factors (interactions among factors). Now we are talking beyond the use of the cause/effect diagram, but the cause/effect diagram started the team off with a large list and from the large list the team can boil down to fewer factors from which the team can experiment.
Let’s say that the team didn’t use the cause/effect diagram. Let’s say that instead of coming up with the hundred possible reasons, one person said that they think that it has everything to do with the fertilization ratios of iron and nitrogen and other minerals. So, the team experiments on different types of fertilizers and find that there is no difference in the amount of grass that the growing under the shade tree. Perhaps the real reason why the grass isn’t growing is because the type of grass that has been planted is not robust to shade. Probably somebody during a brainstorming session using a cause/effect diagram to facilitate the brainstorming session would have come up with the idea of grass seed type. But, the cause/effect diagram utilization step was skipped and therefore no one ever thought of grass seed type. A lot of money could be spent on fertilizer, but if the team would have used the cause/effect diagram as a part of their brainstorming session, somebody would have thought about grass seed type. Less money might have been spent on fertilizer and some of that money might have been spent on different types of grass seed.
