Overview
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.
- For continuous data, the default is 100 observations.
- For attribute data, the default is 25 subgroups.
Note
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.
New control charts
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.
This is a new chart that has just started collecting data. The control limits will continue to update with each new point until 100 points are collected. Then, the control limits will lock.
Control limits are locked.
The circled point is out-of-control when collected, according to the estimated control limits. Because the point is OOC when collected, Real-Time SPC sends an alert if you have set up notifications. For more information, go to My Account.
After more data are collected, this point is now in control.
Existing control charts
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.
- 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.
Changing data requirements during an estimation phase
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.
Important assumptions
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.
