Six sigma is the goal of improving the quality of a product or process. Six sigma uses a repeating cycle of improving the process, monitoring it, and then finding another factor or aspect to improve. Six sigma analytical tools identify areas that require improvement, prioritize them, and help monitor progress toward the new quality standard. The simplest six sigma analytical tools can be broken down into check sheets, charts and diagrams.
Six sigma chart tools include Pareto charts, SPC charts and run charts. The Pareto principle states that 80 percent of all defects are caused by 20 percent of the root causes. Pareto charts are graphs that show which causes result in the greatest number of defects. This is done with each root cause listed from largest to smallest along the X axis. The Y axis shows the percentage of the total increasing as each root cause is added until the total is 100 percent. Those root causes on the far left are the problems for quality improvement.
Statistical Process Control charts are called SPC charts. Run charts and SPC charts plot a variable like weight over time. SPC charts will have an upper and lower acceptable limit while run charts only show the average. Both chart types will vary randomly around an average value. If the chart begins to show a trend in one direction or begins to move toward one of the outward acceptable limits of the SPC chart, the process needs to be brought under control by the six sigma team.
Six sigma analysis can begin with a check sheet. A check sheet can be a check lists or a defect diagrams. Check sheets can be attribute check sheets, location check sheets, and variable check sheets. The check list will include all areas to check or verify before the product is considered good to send to the customer. Defect concentration diagrams involve a picture of the product with checks or x-marks where the defects have been recorded. This provides a visual image as to where the problems are occurring.
Diagrams are used to show all of the causes and factors that affect quality. Cause and effect diagrams list all causes of a bad effect. Cause and effect diagrams can list the bad effects caused by the environment, the organization, and unacceptable measurements. Failure modes and effects analysis, or FMEA, traces all the ways a product or process could fail. It also lists the possible consequences of each type of failure.
Root cause analysis traces the root cause of a specific problem. Each cause is determined by asking why it happened. Each problem is traced back until it has a simple and direct root cause. A single root cause could be a factor in several of the root analysis. For example, lack of documents and drawings could be the root cause of both assembly operators building the product wrong and inspectors not knowing to check for the assembly error.