XbarR chart
From Wikipedia, the free encyclopedia
An XbarR chart is a specific member of a family of control charts. A control chart is a tool used in quality control, specifically SPC or statistical process control, as originally developed by Walter A. Shewhart at Western Electric in 1924 to improve the quality of telephones.
A control chart is a plot of measurements of a product on two special scales, usually located above and below each other and running horizontally. For the specific case of the XbarR chart, the top chart has a centerline represented by Xbar, which is simply the sample average. The top and bottom borders are also known as the upper control limit or UCL and the lower control limit or LCL, which are represented by plus and minus three standard deviations from the mean. The bottom chart has the range of each subgroup plotted in a similar manner. The significance of subgroups is explained below.
The purpose of any control chart is to help determine if variations in measurements of a product are caused by small, normal variations that cannot be acted upon, or by some larger special cause that can be acted upon or fixed. The type of chart to be used is based on the nature of the data.
The XbarR chart is normally used for numerical data that is grouped in subgroups in some logical manner, for example 3 games of bowling that occur on one night. This allows the night to be considered as a unit, so a special cause such as a slippery floor or a sick bowler will be more obvious as a point on the chart.
