Real-Time SPC calculates the center line and control limits from known historical process parameters or
from estimated process parameters. If you have known historical process parameters, specify
them either in the product/process/measure setup or at the station level.

Whether you use known historical parameters or estimated from the process data, it is important that the calculations represent a stable process. Using more data helps ensure representative process parameters and control limits.

You can specify the amount of data to use for the control limit calculations. For
continuous control charts, Real-Time SPC uses X observations. For attribute control
charts, Real-Time SPC uses X subgroups.###### Note

- For continuous data, the default is 100 observations.
- For attribute data, the default is 25 subgroups.

This calculation includes missing observations if they are present during the parameter estimation phase. Also, if your incoming data enter the system as a batch and includes more data than necessary, Real-Time SPC uses the entire batch to calculate the control limits. For more information on how to increase or decrease the amount of data in the calculations, go to Control limit options.

A new control chart enters the parameter estimation phase as indicated by station or process settings. For a new chart, without data, Real-Time SPC recalculates the control limits as new data are added. Real-Time SPC ends the estimation phase and locks the control limits when the user-defined number of observations/subgroups has been reached.

During estimation, as control limits vary, the control status of the points may also
vary with each new data point.

An existing control chart enters the parameter estimation phase as indicated by station or process settings, or when a recalculation is initiated from the Process Quality Snapshot.

For an existing chart with data, Real-Time SPC recalculates and locks the control limits based on previous data.

- If the amount of existing data is greater than the user-defined number of observations/subgroups, Real-Time SPC creates a new stage and uses the last X number of observations/subgroups to calculate new control limit parameters for the incoming data.
- If the amount of existing data is less than the user-defined number of observations/subgroups, Real-Time SPC includes only these data in the control limit calculation.

It is possible, but not recommended, to terminate the estimation phase before the required amount of data has been collected. The terminated estimation phase will create a new phase using the data available before termination.

For example, suppose the data requirements estimate parameters using 50 subgroups. During data collection, you realize the process is very stable and the data are representative after 35 subgroups.

- From the Process Quality Snapshot , open the Control Charts tab.
- Select the icon, then select End Estimation Phase.

Control charts assume that you enter all your data in time order. If you violate this assumption, the points that were added out of order will be inserted in proper timer order. However, if there are multiple stages in the control chart, the points that were added out of order are inserted in proper time order with a separate stage and control limits.

Also, if two or more subgroups are entered with the exact same timestamp but different action or specification limits, Real-Time SPC uses the settings associated with the highest system identifiers.