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

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

Quality control is important to any business. Quality control is the process of making sure that the product or service that is being offered by your company is of the utmost quality. Many times quality is measured numerically and charted so that the levels of quality can be evaluated numerically. As the process of Six Sigma is highly invested in the quality of its product, it is only natural that measurements of that quality, such as are illustrated in a control chart, be part of the Six Sigma method.

What is a control chart?

A control chart is a tool that is used to determine whether or not a process is being controlled at a level that is satisfactory to management. Statistical control is measured on a graph where various aspects of the product are plotted and the resulting pattern should reveal whether or not there is variation beyond an acceptable level. Ultimately a control chart is used to measure that is and what is not in control.

A control chart is just one of several different tools that can be used as part of a quality control measurement. A control, chart should consist of points (alluded to above) that indicate an average measurement of some aspect of the products' quality (appearance, performance, durability, etc.). A center line is drawn through the chart to indicate where the points in the graph are at the mean. There are also upper and lower limits on the graph. Upper and lower limits are terms used to describe the points in the chart where performance above or below those levels is unacceptable. If the product evaluation yields a point that is outside of the control limits, then that product does not meet the quality assurance specifications. These upper and lower limits are usually drawn two standard deviations from the center line. Standard deviations indicate a percentage of products that are also likely to conform to the performance specifications at the mean measures average levels. In other words, not all products are sampled so the ones that are selected for intensive quality testing are deemed representative of the whole group. Two standard deviations is the accepted variance level that is used for the group as a whole.

How is a control chart used in Six Sigma?

The goal of Six Sigma is to improve all processes to meet or exceed specific levels of quality. Six Sigma aims to achieve a level of quality where the levels of defective products are below 3.4 defects per (one) million opportunities. Six Sigma uses the control chart to measure their quality performance and used the standard deviations of the control chart to see whether or not they are able to achieve their goal of a low number of defects. A performance level point that is charted on the control chart in between two standard deviations from the center line, meets the requirements of Six Sigma.

The process of collecting and interpreting these performance points on the graph can differ greatly depending on the theories of the quality assurance team and the nature of the product itself. Some manufacturing companies have the entire control chart process fully automated. A computer makes a measurement of quality at various points in the process of production. The resulting mark is them plotted on a control chart. Defects are plotted outside of the standard deviations of acceptable quality and the computer automatically knows to stop production or to run a diagnostic to find and correct any problems that are occurring on the production line. Most every company performs manual quality assurance tests. Performance and quality are measured and assigned a score. That score is plotted on the control chart and in accordance to the Six Sigma method if the plotted point is outside of the acceptable range the product is rejected and the process of correcting the problem begins.