![]() The three charts above are used when plotting continuous data. Take a few minutes and review the data before you begin to analyze. Copying/pasting data from spreadsheets or paper can seem simple but numbers can get missed, decimals moved, etc. Make sure that the data in Minitab (Excel, Sigma XL, etc.) is correct. When the S chart is in control, then an out-of-control condition found in the Xbar chart is related to changes in the process center.Īs with any software, the results will only be as good as the data selected. The S chart must be in control in order to properly interpret the Xbar chart. The S chart plots the range of the subgroup standard deviations and is used to determine whether the process variation is in control. The Xbar charts determine whether the process center is in control. The Xbar chart plots the mean of the each subgroup. These charts are used to verify process control before assessing capability such as Cpk, Ppk, Cp, Pp, or Cpm. ![]() There are a few commonly used charts to assess process control If the S chart is out of control, then the control limits on the X-bar chart may be inaccurate and exhibit Type I or II error. The S chart is used to review the process variation which must be in control to correctly interpret the Xbar chart. The control limits of the Xbar chart are calculated with the inputs of the process spread and mean. This control chart, along with I-MR and Xbar-R, are used in measuring statistical process control and assessing the stability of a process. A Xbar-S (Sigma) control chart is often used together to plot the process mean (Xbar) and subgroup standard deviations ( S) over time for continuous data.
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