False alarms
Although the individual patterns that we use as triggers occur rarely in a process that is in control, they do occur occasionally. Indeed, the proportion of values from a stable process that triggers each of the criteria is typically about 1 in 200, so if all five criteria are used, a reasonable number of false alarms will occur.
Process in control
The following diagram shows a series of measurements from a stable process (a normal distribution with mean 14.5 and standard deviation 1.0).
As before, red dots will denote values that are exceptional. Clicking on a red dot will show which of the criteria has been triggered. Click the button 1 extra several times to record other values from the process. To speed up the sampling, use the other buttons to generate more values at a time.
Clearly a single exceptional value is not conclusive proof that the process is out of control. However it is appropriate to examine carefully the operation of the process to look for an assignable cause for this value (and adjust the process if such a cause is discovered). And a series of such values does indicate that the process is out of control.