Extreme Point Condition This process is out of control because a point is either above the UCL or below the UCL.This is the most frequent and obvious out of control condition and is true for all control charts.
Two of Three Consecutive Points in Zone A or Outside Zone A The probability of having two out of three consecutive points either in or beyond zone A is an extremely unlikely occurrence when the process mean follows the normal distribution. Thus, this criteria applies only to charts for examining the process mean.X, Y, and Z are all examples of this phenomena.
Runs Above or Below the Centerline The probability of having long runs (8 or more consecutive points) either above or below the centerline is also an extremely unlikely occurrence when the process follows the normal distribution. This criteria can be applied to both and r charts. Example X above shows a run below the center line.
Linear Trends The probability of 6 or more consecutive points showing a continuous increase or decrease is also an extremely unlikely occurrence when the process follows the normal distribution. This criteria can be applied to both and r charts. X and Y are both examples of trends. Note that the zones play no part in the interpretation of this out of control condition.
Oscillatory Trend The probability of having 14 or more consecutive points oscillating back and forth is also an extremely unlikely occurrence when the process follows the normal distribution. It also signals an out of control condition. This criteria can be applied to both and r charts.
Avoidance of Zone C The probability of having 8 or more consecutive points occurring on either side of the center line and do not enter Zone C is also an extremely unlikely occurrence when the process follows the normal distribution and signals an out of control condition. This criteria can be applied to charts only. This phenomena occurs when more than one process is being charted on the same chart
Run in Zone C The probability of having 15 or more consecutive points occurring the Zone C is also an extremely unlikely occurrence when the process follows the normal distribution and signals an out of control condition. This criteria can be applied to charts only. This condition can arise from improper sampling, falsification of data, or a decrease in process variability that has not been accounted for when calculating control chart limits, UCL and LCL.
