Cause-and-Effect Diagrams as Total Quality Tool

A team typically uses a cause-and-effect diagram (see Figure 15.8) to identify and isolate causes of a problem. The technique was developed by the late Dr. Ishikawa, a noted Japanese quality expert, so sometimes the diagram is called an Ishikawa diagram. It is also often called a fishbone diagram because that is what it looks like.

In his book Guide to Quality Control, Ishikawa explains the benefits of using cause-and-effect diagrams as follows:3

  • Creating the diagram itself is an enlightening, instruc­tive process.
  • Such diagrams focus a group, thereby reducing irrel­evant discussion.
  • Such diagrams separate causes from symptoms and force the issue of data collection.
  • Such diagrams can be used with any problem.

The cause-and-effect diagram is the only tool of the seven tools that is not based on statistics. This chart is simply a means of visualizing how the various factors associated with a process affect the process’s output. The same data could be tabulated in a list, but the human mind would have a much more difficult time trying to associate the factors with each other and with the total outcome of the process under investi­gation. The cause-and-effect diagram provides a graphic view of the entire process that is easily interpreted by the brain.

Suppose an electronics plant is experiencing solder­ing rejects on printed circuit (PC) boards. People at the plant decide to analyze the process to see what can be done; they begin by calling together a group of people to get their thoughts. The group is made up of engineers, solder machine operators, inspectors, buyers of materials, production con­trol specialists, and others. All the groups in the plant who have anything at all to do with PC boards are represented, which is necessary to get the broadest possible view of the factors that might affect the process output.

The group is told that the issue to be discussed is the solder defect rate and that the objective is to list all the fac­tors in the process that could possibly have an impact on the defect rate. The group uses brainstorming to generate the list of possible causes. The list might look like Figure 15.7.

The group developed a fairly comprehensive list of fac­tors in the PC board manufacturing process—factors that could cause the effect of solder defects. Unfortunately, the list does nothing in terms of suggesting which of the 35 fac­tors might be major causes, which might be minor causes, and how they might relate to each other. This is where the cause-and-effect diagram comes into play. Ishikawa’s genius was to develop a means by which these random ideas might be organized to show relationships and to help people make intelligent choices.

Figure 15.8 is a basic cause-and-effect diagram. The spine points to the effect. The effect is the “problem” we are interested in—in this case, machine soldering defects. Each of the ribs represents a cause leading to the effect. The ribs are normally assigned to the causes considered to be major fac­tors. The lower level factors affecting the major factors branch off the ribs. Examine Figure 15.7 to see whether the major causes can be identified. These causes are assigned to the ribs.

Six major groupings of causes are discernible listed as follows:

  1. The solder machine itself is a major factor in the process.
  2. The operators who prepare the boards and run the sol­der machine are also major factors.
  3. The list includes many items such as parts, solder, flux, boards, and so on, and these can be collected under the word materials, which also appears on the list. Materials is a major factor.
  4. Temperature within the machine, conveyor speed and angle, solder wave height, and so on, are really the methods (usually published procedures and instruc­tions) used in the process. Methods is a major factor.
  1. Many of these same items are subject to the plant’s methods (how-to-do-it) and measurement (accuracy of control), so measurement is a major factor, even though it did not appear on the list.
  2. The cleanliness, lighting, temperature and humidity, and quality of the air we breathe can significantly af­fect our performance and thus the quality of output of processes with which we work. We will call this major factor

The designated six major factors, or causes, are those that the group thinks might have an impact on the quality of output of the machine soldering process: machine, operator, materials, methods, measurement, and environment. The cause-and-effect fishbone diagram developed from this in­formation has six ribs, as shown in Figure 15.9.

Having assigned the major causes, the next step is to as­sign all the other causes to the ribs they affect. For example, machine maintenance should be assigned to the Machine rib because machine performance is obviously affected by how well or how poorly the machine is maintained. Training will be attached to the Operator rib because the degree to which operators have been trained certainly affects their ex­pertise in running the machine. In some cases, a possible cause noted on the list may appropriately branch not from the rib (major cause) but from one of the branches (con­tributing cause). For example, solderability (the relative ease—or difficulty—with which materials can be soldered) will branch from the Materials rib because it is a contribu­tor to the materials’ cause of solder defects. An important cause of poor solderability is age of parts. So age of parts will branch not from Materials but from solderability. Study Figure 15.10 to get a graphic sense of the relationships de­scribed in this paragraph.

Figure 15.10 is the completed fishbone diagram. It presents a picture of the major factors that can cause sol­der defects and, in turn, the smaller factors that affect the major factors. Examination of the Materials rib shows that there are four factors directly affecting materials in regard to solder defects: the parts themselves, the handling of the materials, and the solder and flux used in the process. The chart points out that contamination can affect the solder’s performance and also that the big issue affecting the parts is solderability. In this case, the branches go to three levels from the rib, noting that solderability can be affected by the vendor supplying the parts, storage of the parts before use, and age of the parts.

Now you may say, “The diagram didn’t configure itself in this way. Someone had to know the relationships before the diagram was drawn, so why is the diagram needed?” First, picture these relationships in your mind—no diagram, just a mental image. If you are not familiar with the process used in the example, pick any process involving more than two or three people and some equipment, such as the process of an athletic event. If you try this, you will probably find it virtually impossible to be conscious of all the factors com­ing into play, to say nothing of how they relate and interact. Certainly, the necessary knowledge and information already existed before the 35 factors were arranged in the cause-and- effect diagram. The key to the diagram’s usefulness is that it is very possible that no one individual had all that knowledge and information. That is why cause-and-effect diagrams are normally created by teams of people widely divergent in their expertise.

The initial effort by the team is developing the list of possible factors. This is usually done using brainstorming techniques. Such a list can be made in a surprisingly short time—usually no more than an hour. It is not necessary that the list be complete or even that all the factors listed be truly germane. Missing elements will usually be obvious as the diagram is developed, and superfluous elements will
be recognized and discarded. After the list has been com­piled, all the team members contribute from their personal knowledge and expertise to assemble the cause-and-effect diagram.

The completed diagram reveals factors or relationships that had previously not been obvious. The causes most likely responsible for the problem (solder defects) will normally be isolated. Further, the diagram may suggest possibilities for action. It is conceivable in the example that the team, be­cause it is familiar with the plant’s operation, could say with some assurance that solderability was suspected because the parts were stored for long periods of time. They might rec­ommend that, by switching to a JIT system, both storage and aging could be eliminated as factors affecting solderability.

The cause-and-effect diagram serves as an excellent re­minder that the items noted on it are the things the com­pany needs to pay attention to if the process is to continually improve. Even in processes that are working well, continual improvement is the most important job any employee or team can have. In today’s competitive global marketplace, it is truly the key to survival.

Source: Goetsch David L., Davis Stanley B. (2016), Quality Management for organizational excellence introduction to total Quality, Pearson; 8th edition.

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