Wednesday, September 10, 2008

Constructing Cause-and-effect Diagrams

Have you ever had a quality problem that couldn't be quantified through the use of statistics? The relationship between a problem's effect and its cause is sometimes obscure. To solve the problem, you may need to examine the entire process, identifying all the potential problem sources before you can determine the root cause. When visually displayed, this problem-identification process is called cause-and-effect diagramming.

Cause-and-effect diagrams are also known as "wishbone" or "fish" diagrams because of their shape. A cause-and-effect diagram has a central "back bone" with "ribs" branching off. The process used to create the diagram is known as flowcharting.

Cause-and-effect analysis is frequently completed by a team since specialists in many areas or departments may be required to provide input into their part of the process. During a project meeting or conference, the project team will construct the diagram, starting with the problem and working backward to the beginning of the project or process.

In cause-and-effect diagramming, there are three possible methods for identifying causes:
  1. the random method
  2. the systematic method
  3. the process analysis method
The random method, during which team members randomly cite problems and probable causes, is a somewhat haphazard approach and may not identify all problem categories. However, it is useful as a general trouble-shooting technique and can help get the group in problem-solving mode.
The other two methods, systematic and process analysis, are more structured and rational in the identification of causes. These are used mostly by engineers and technicians.

The systematic method focuses analysis on one category at a time. Each category is examined in descending order of importance after the primary one is addressed.

The process analysis method looks at a production process identifying each sequential step, and the categories and causes for each step, one at a time.

When the cause-and-effect diagram has been constructed, the team or project manager can then suggest changes to the potential problem-causing areas. A series of experiments or additional statistical analysis may be necessary to determine the primary or root cause of a given problem.

The next step is to decide what corrective action is necessary. In the process of identifying the problem, the team suggests the desired outcome. By turning the diagram around, you can determine what impact the desired outcome will have on each of the listed categories and causes. In some cases you may find corrective action is required in one or all of the categories.

A machine in a process may require finer adjustment or even replacement if worn out. A newer machine may offer a leap in technology that may eliminate present quality control problems through greater automation or internal computer control processes.

The production or process method may require fine tuning, involving additional substeps before completion of a task. The layout of a process may also need reorganizing. Duties and responsibilities may need increased emphasis.

Stricter inspection and handling of raw materials may be necessary to ensure that they are of high standard. Improved selection of raw materials may also be required.

Changes to the measurement process may be necessary, such as more accurate measurements, increased frequency of inspection, and introduction of new statistical methods for analysis.

Perhaps current personnel are inadequately trained in both their job and in quality control. Upgrading job skills may be necessary. It is also possible that declining quality is due to poor work habits or boredom with the job.

Once the root cause of the problem has been identified and the effects of the desired outcome on other areas of the project have been studied, appropriate corrective action is taken. At this point, change requests are processed and safeguards set in place to prevent future recurrences.

Even when a problem has multiple sources, you will find cause-and-effect diagrams are invaluable in pinpointing them all. Flowcharting with cause-and-effect diagrams is an effective way to conceptualize causes to a problem in a project or process. Once flowcharted, the problem can be further analyzed using other analytical tools. Ultimately, corrective action and protective safeguards are applied to solve the problem and prevent future recurrences.

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