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What is a control chart and how is it used in Six Sigma?

Control charting is a tool used to monitor processes and to assure that they remain stable. The control chart is also known as the 'Shewhart chart' or the 'process-behavior chart'.It is a statistical tool often used in Six Sigma and is intended to assess the nature of variation in a process and to facilitate forecasting and management. The control chart is one of the seven basic tools of quality control, which also include the histogram, Pareto chart, check sheet, cause-and-effect diagram, flowchart, and scatter diagram.
Control charts were proposed by Walter A. Shewhart in 1924 while working for Bell Labs.They recognized the need to reduce the frequency of failures and repairs because their telephony transmission systems' amplifiers and other equipment had to be buried underground.Shewhart stressed that bringing a production process into a state of statistical control, where there is only common-cause variation and keeping it in control, is necessary to predict future output and to manage a process economically.Dr. Shewhart concluded that while every process displays variation, some processes display controlled variation that is natural to the process, while others display uncontrolled variation that is not present in the process causal system at all times.

Control charts help distinguish process variation due to assignable causes from those due to unassignable causes.Both these types of process variation are charted on a control chart.Assignable causes or special causes are meaningful factors of a process and are not always present or normal.These types of causes can be avoided and should be investigated.Unassignable causes are also known as common causes or chance causes.These are factors that occur by chance.They are not always present, but are normal and expected within a process.They are unavoidable and inherent in a process.
The elements present on a control chart include:
1. a central line: drawn at the process mean.
2. an upper warning limit: drawn two standard deviations above the center line.
3. an upper control limit or upper natural process limit: drawn three standard deviations above the center line.
4. a lower warning limit: drawn two standard deviations below the center line.
5. a lower control limit or lower natural process limit: drawn three standard deviations below the center line.
6. process values plotted on the chart.
If all process values are plotted within the upper and lower control limits and no particular tendency is noted, the process is considered stable and is referred to as "In Control". If the process values are plotted outside the control limits or show a particular tendency, however, the process is referred to as "Out Of Control"
The process of creating a Control Chart is as follows:
1. Select the process you would like to chart
2. Determine your process sampling plan
3. Collect data from your process
4. Calculate the control chart specific statistics
5. Calculate your control limits
6. Construct your control chart (software options)
If a point falls outside of the limits established for a given control chart, those responsible for the underlying process are expected to determine whether a special cause has occurred. If one has, then that cause should be eliminated if possible.
In summary, control charts are a necessary tool in the Six Sigma methodology because of their role in assessing the nature of variation in any particular process.They aide in forecasting improvement programs as well as monitoring existing processes to ensure that they remain stable.

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