Calculating Control Limits

Control Limits can be calculated directly from an SPC Chart. When performing a calculation, the application uses a specific date range to estimate sigma. This sigma estimate is derived from the average range (R-bar) or standard deviation (s-bar), based on the selected calculation style. Once sigma is estimated, the application applies standard SPC formulas to define the control limits for the X and R charts.

Calculating control limits involves a certain level of risk. The process must be stable and in a desired state during the selected calculation period. If the process is not stable at that time, the resulting control limits are generated to ensure reliability. Control limits can be applied using these methods:

  • Default Limits
  • Set Limits
  • Calculated Limits

Default Limits

Default Limits are defined for each specification (product). These limits are automatically applied to each new cut‑by combination that includes that product when the application collects data for the first time.

Note: If the application has already recorded data for variable and product combination, default control limits are added, however, not applied. The default control limits apply only when the application collects data for the first time for the variable and product.

Set Limits

Set Limits can be defined a user or administrator. The application uses set limits to override default limits when more tailored control is required for subsets of the process.

Note:  Once control limits are added, they only apply going forward; the system does not retroactively evaluate past data against these newly set limits.

Calculated Limits

Calculated Control Limits are determined by the SPC engine using a specified date range that represents a stable process. To set the starting time, click on the point in the chart that you wish to use. Select Calculate on the Limits tab. A date is required to set the end of the stable period.