How to apply custom limits
This help page describes how to use the custom limits feature of the Craydec Control Charts.
When custom limits are needed
Usually, control charts are used by determining the control limits (Upper and Lower Control Line for data, Upper and Lower Control Line for variance) from the data. However, sometimes there arises a need to apply control lines other than from data.
Custom limits are typically needed when you have the process in control, and you have already determined the control line values. Sometimes you want to use a non-standard formula, or you might use this feature to track data against specification limits instead of control limits.
How to do it
There are two ways to use custom limits.
1) Static custom limits (limits from settings)
2) Dynamic custom limits (limits from data)
Static (limits from settings)
Static custom limits are fast and easy to use, as you just enter the custom limits to the visual’s settings. However, you can enter only one set of limits, so if you are using subgrouping (steps), the same set of custom limits is applied to all of the subgroups (steps).
Enable custom limits
Set Format > Custom Limits : On
Select Format > Custom Limits > Limit Type : static
Select which limit you want to replace:
Format > Custom Limits > Enable custom (UCL/CL/LCL/UCLv/LCLv)
Enabling a limit will override it’s value from settings. Disabling a limit will use the calculated value. If you do not want to show some value, hide it from section ‘Control Lines’.
Enter custom limits
Enter your custom limits to the visual’s settings.
Format > Custom Limits > UCL for data / CL for data / LCL for data / UCL for variance / LCL for variance
Notes
The custom limit text fields appear only after you have enabled the custom limits, selected static limit type, and enabled the line to be overridden.
If you want to enter a zero value, insert a non-zero value first, then change it to zero.
Dynamic (limits from data)
Dynamic custom limits are a bit more difficult to set up that the static limits, but it allows you to drive the visual from your data. This makes it possible to control the visual from your backend, for example by (re-)calculating the limits in the database.
Enable custom limits
Set Format > Custom Limits : On
Select Format > Custom Limits > Limit Type : dynamic
Select which limit you want to replace:
Format > Custom Limits > Enable custom (UCL/CL/LCL/UCLv/LCLv)
Select data
Select variables/columns for data roles (Custom UCL & LCL, Custom UCL & LCL for variance)
Requirements for data
There are some special considerations for the data used for custom lines.
1) Data type must be number.
2) The first line of a subgroup (step) must contain the custom line
values. If a value is not found the control lines are not drawn. To be on the safe side, repeat data for each line of a subgroup (step). Same logic applies to facets.
3) For S-, and R-charts the first line of subgroup (sample) must contain the custom line value for a datapoint. If a value is missing, the control line is now drawn.
When using custom limits with steps (subgroups) and facets, it’s your best bet to provide the limit values for all datapoints to make sure the values appear correctly.
Example data
When using subgrouping (steps), subgroup data role must be used to indicate a new subgroup. The same variable/column that is used for custom lines, can be used. Each change in value starts new subgroup.
In the picture above, the black rectangles indicate a new subgroup (step) start, and the limits used for the subgroup.
Examples
Download example report, and enter a valid license code to see the custom limits in action.