Control limit options

Learn about control limit options in Real-Time SPC.

In This Topic

Setting calculation methods for new charts
Use recent data to recalculate control limits
Use a range of data to recalculate parameters for the control limits
Turn off the control limits on a chart

Setting calculation methods for new charts

For a new control chart, Real-Time SPC calculates the center line and control limits from estimated process parameters. You can specify the amount of data to use for the control limit calculations.

Real-Time SPC provides several ways to change the calculation method based on which charts you want to customize. After the center line and control limits of a control chart have been calculated, they are set and locked. These lines will remain until a new calculation is initiated. For more information on control limit calculations, go to Control limit calculation details.

How to change system preferences

To specify the amount of data to use for parameter estimates for all new charts, update the system preferences. These settings affect the default values in the measure setup settings.

Select Analysis Preferences , then navigate to Control Charts > Control Limit Settings.
Select to open the options.
For continuous data, the default is 100 observations. For attribute data, the default is 25 subgroups. Change these settings if you want to include more or less data in the calculations.

For more information, go to Control chart preferences.

How to change product/process/measure setup

To specify the amount of data to use for parameter estimates for all new charts of a particular measure, go to the control chart settings of the measure. These settings affect the default values in the station setup settings.

Go to the Components page and select a product. Then select the process flow step of your product.
Open the process step and go to the Process Summary section.
Select Additional Settings and then select Control Charts.
In Control limit calculation method, choose the method to calculate control limits.

For more information, go to Specify control chart settings for each measure.

How to change station setup

To specify the amount of data to use for parameter estimates for a particular station, go to the station settings of the measure. A change here will trigger a new calculation.

Go to the Components page and select a station.
For the product and process to change, select Configure.
Select Edit for each measure to choose the data source.
Select the Control Chart tab.
In Control limit calculation method, select the method to calculate control limits.

For more information, go to Example of specifying the method for calculating control limits for each station.

Use recent data to recalculate control limits

Open the Process Quality Snapshot which provides the process control details associated with a measure.

Open the Control Charts tab.
Select the icon, then select Recalculate control limits.
Select which measures and stations to apply the new control limit calculations. The parameter estimates will use the current data points, up to the amount of data specified by your station settings. Make sure you have enough data to accurately recalculate the center line and control limits before initiating a recalculation.
Note
While it is common to use control limits that are calculated from recent data, you may prefer to find a time of stability in the process and use the historical parameters from that time to set control limits.

Use a range of data to recalculate parameters for the control limits

Open the Process Quality Snapshot which provides the process control details associated with a measure.

Open the Control Charts tab.
Select the icon, then select Advanced options.
Select Edit Parameters. While in edit mode, you can do one of the following.
- Enter new parameters directly based on your process knowledge.
- Select a stable range of data from the control chart to estimate the parameters.
You can preview the new limits and decide whether to adjust the parameters further before you update the new control limits.
Select Apply when you are ready to use the updated parameters.
Specify which stations to update with new control limits.

Explore your data to find historical parameters

Use the date selectors to specify the start and end times for the control chart. Then, select a range of data on the chart when the process was stable.

These parameters are calculated from the selected range of data.

Turn off the control limits on a chart

You can turn off the control limits for any control chart. When you turn off the limits, only Test 2, Test 3, and Test 4 are available.

When you turn off the control limits during product and process setup, they are turned off for all the stations.

Go to the Components page and select a product. Then select the process flow step of your product.
Open the process step and go to the Process Summary section.
Select Additional Settings and then select Control Charts.
In Control limits, select Do not use control limits.

When you turn off the control limits from a station setup, the change is to this station only.

Go to the Components page and select a station.
For the product and process to change, select Configure.
Select Edit for each measure to choose the data source.
Select the Control Chart tab.
In Control limits, select Do not use control limits.