Alarming Considerations in the Statistical Data Block

The Statistical Data block supports out of control alarming on the following limits:

• XBAR, Range, and Standard Deviation control limits.  
• Warning limits for XBAR plots.
• Trend Alarm.  
• Length of Runs Alarm.
• Crossings of the Mean Alarm.

Statistical Data block alarms are based on groups. Group alarming occurs if a user-defined group exceeds any limit. These alarms include the following limits:

Control Limits – defines the limits as within plus or minus three (+/- 3) standard deviations (also called sigma).

Warning Limits – defines the limits as within plus or minus two (+/- 2) standard deviations (XBAR plot only).

At run-time, the Statistical Data block calculates all control limits and statistical values after collecting the first n groups, where n is the number of groups specified in the block's dialog box.

You can specify alarm limits through Data links to the following fields. The block supports both A_ and F_ versions of these fields.

If you manually input limit values, verify that the values are very close to the ones the block would calculate. Otherwise, the validity of statistical alarms and control charts is questionable. You should also select NEVER for the block's Auto Limits and Auto Calculations fields when you manually enter limit values. This selection lets the block calculate alarm limits, XBAR, R, and S values based upon defaults and lets you enter values at any time before or after startup. Operators can also change the setting of Auto Calculations or Auto Limits through Data links to the fields A_AUTOX and A_LCALC, respectively.