Control charts use Control Limits to determine whether the examined process is under control or not. The Upper Control Limit (UCL) determines how high value can be and the process is still considered to be in control. The Lower Control Limit (LCL) determines to lower limit of stable process. When charting the value, they are sometimes referred to as Upper Control Line and Lower Control Line.
Control chart also includes a value to indicate the center of the process value, usually average. This value is called Center Line (CL).
Some control charts include a secondary chart (for example the ImR-chart). The secondary chart plots the change. This variation chart include also three different values to indicate the center, the high limit, and low limit. These are called Center line for variation (CLv), Upper Control Limit for variation (UCLv), and Lower Control Limit for variation (LCLv).
The UCL and LCL indicate a limit of 3-sigma values from the center. In addition of these 3-sigma lines, it is sometimes useful to calculate and draw 1-sigma, and 2-sigma lines on both sides of the center. By design, these values are close to the process average than the 3-sigma lines.
The gist of the control charts is that three sigma distance is calculated from the variance indicated by the data. This value is added to the process average (CL) to get the UCL, and it is subtracted from process average (CL) to get the LCL.
However, the actual calculation is more difficult that the high level description. If you are interested in details, or implementing your own solution, you should read our blog post on the issue.
The formulas can be found from wikipedia.
Craydec Control Charts support using all the above mentioned lines, where applicable.
Individual lines can be enabled and disabled from the visual’s settings.
Format > Control Lines
By default all of the data points are used to calculate the limits. This can be changed by selecting how many points from beginning (calculated from the beginning of a step) is used to calculate the lines.
Format > Control Lines > Use all points in calculation
When disabling the settings, you can set how many points to use.
See the separate page on recalculating the limits.
You can by pass the visual calculation of the limits completely, and supply your own values for the lines.
See the separate page on custom lines