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Cause-and-Effect Diagrams
As one gains an understanding of the application of the techniques found in the sigma toolbox, one will also realize that there is a clear link between the objectives of Six Sigma and the tools that enable a project. In the case of reducing cycle times, both the DPM and VAP map are extremely helpful in pinpointing dead time in a process. COPQ calculations help team members to quantify the expense associated with process variation and begin the task of reducing costs by stabilizing activities. Because Six Sigma is based on the dynamics of causal relationships and recognizes that the quality of an output is contingent upon all of its inputs, use of cause-and-effect diagrams must be part of any project.The cause-and-effect diagram (also known as a fishbone or Ishikawa diagram) was first developed in Japan and is a carryover from the total quality management and lean manufacturing schools of thought. The diagram itself resembles a fish skeleton, with the problem (effect) placed at the head of the spine on the right-hand side. Each bone of the skeleton represents an input to the process (cause) that may be responsible for the problem. The headers of the bones that connect to the skeleton are categorized by people, process, technology, physical plant and materials. As each category is analyzed in detail, more bones are added to the skeleton, thereby allowing team members to list inputs in an organized fashion.Figure 9.5 is a cause-and-effect diagram that was prepared by a Taiwanese manufacturer of measuring equipment. Recent shipments received from the company's number one domestic supplier of components and assemblies have shown an inordinate increase in defects. Although never a problem in the past, these supplier shipments have caused lines to go down and work-in-process inventories to grow to unacceptable levels. Fortunately, the manufacturer employs an in-house engineer at the supplier's facility. In conjunction with the supplier and the manufacturer, the engineer organized a series of brainstorming and analysis sessions to eradicate the problem. Part of the analysis was a fishbone diagram that uncovered some interesting facts.Figure 9.5: Cause-and-Effect Diagram for Defective Parts
One of the benefits of a fishbone diagram is that it logically organizes inputs and allows for in-depth analysis of each category. In the case of the Taiwanese company, the first "bone" that drew attention was physical plant. The supplier confirmed that it had purchased a new high-end production line in the last three months and had experienced certain "problems" with the line. The equipment employed new technology and was intended to be a multi-purpose line. As the team noted, this core change in production spilled over into other areas of the business that contributed to the defective parts problem.Breakdown of the people section of the diagram revealed that the company that sold the equipment to the supplier had not provided the technical support or training committed to in the contract. Because the equipment utilized a new technology with a different manufacturing process, production personnel could not apply much from their past experience to improve current output. Listed as causes under each header, the combination of the new technology, different production process and untrained personnel caused the line to run out of gauge and produce defective parts.In addition to the problems caused by the installation of the new equipment, it was also revealed that the supplier had changed a vendor right around the same time that the equipment was brought in. There was a clear breakdown of communication between the purchasing and production departments at the supplier, and all agreed that it was probably not a good idea to change vendors during the same period a new line was being installed. Nevertheless, the diagram exposed that the vendor had its own quality issues that exacerbated the internal problems alluded to above.As the example of the Taiwanese manufacturer illustrates, preparation of a cause-and-effect diagram has a great deal of value when a team must categorize and analyze process inputs. When used in conjunction with other sigma tools (brainstorming and affinity diagrams, for example), this technique is especially helpful in identifying problem areas. It is important to note, however, that the fishbone diagram not only isolates problems by category but also identifies cause-and-effect relationships between the categories themselves. Without question, the Taiwanese company's problems started with the purchase of the new equipment. However, the impact of the purchase quickly trickled into the process and people categories. Although disconcerting, the ability of the Ishikawa diagram to identify both first- and second-order causal relationships will ultimately help a team to resolve the effect on the entire process, defective parts.Because of its ability to pinpoint cause-and-effect relationships, the Ishikawa diagram is used frequently during the analyze stage of a Six Sigma project. While the company under analysis still has a great deal of work to do, it can continue through the sigma methodology using other tools that will lead to better process design and performance.
