Control charts, also known as Shewhart charts or Standard Control Charts, are tools used in Statistical Process Control (SPC) to monitor and analyze processes. The goal is to keep them in a stable state and ensure they are within the desired specifications. Control charts are one of the key tools of Six Sigma and other quality improvement methods.
XP-R (X-Bar and R Chart): This type of control chart is used to monitor the mean (X-bar) and variance (R) of a process. On the XP-R plot, the samples from the sampling are shown and the mean and variance of each sample is compared to the control limits.
XM-R (X and Moving Range Chart): XM-R chart tracks the individual values (X) of the process and the moving range (Moving Range, MR) between two consecutive samples. This type of control chart is used when it is not possible to obtain enough samples to create an XP-R chart.
XP-S (X-Bar and S Chart): The XP-S chart is similar to the XP-R chart, but instead of variance, it tracks the standard deviation (S) within the sample. This type of control chart is used when it is important to monitor process variability.
XI-MR (Individuals and Moving Range Chart): The XI-MR chart tracks the individual values (Individuals, I) of the process and the moving range (Moving Range, MR) between two successive values. This type of control chart is used when we do not have enough samples to create an XP-S chart and monitor individual values.
Each of these control charts provides a way to visually monitor and assess the stability of a process and identify deviations or anomalies that could lead to poor performance or quality issues. They help organizations keep processes under control and take corrective actions to improve the performance and quality of products or services.