2.
Measurement Process Characterization
2.2. Statistical control of a measurement process


Bias and variability are controlled by monitoring measurements on a check standard over time 
Bias and longterm variability are controlled by monitoring measurements
on a check standard over time. A change in the measurement on the check
standard that persists at a constant level over several measurement
sequences indicates possible:
A change in the variability of the measurements on the check standard can be due to one of many causes such as:


The control procedure monitors the progress of measurements on the check standard over time and signals when a significant change occurs. There are two control chart procedures that are suitable for this purpose.  
Shewhart Chart is easy to implement  The Shewhart control chart has the advantage of being intuitive and easy to implement. It is characterized by a center line and symmetric upper and lower control limits. The chart is good for detecting large changes but not for quickly detecting small changes (of the order of onehalf to one standard deviation) in the process.  
Depiction of Shewhart control chart 
In the simplistic illustration of a Shewhart control chart shown below,
the measurements are within the control limits with the exception
of one measurement which exceeds the upper control limit.
 
EWMA Chart is better for detecting small changes  The EWMA control chart (exponentially weighted moving average) is more difficult to implement but should be considered if the goal is quick detection of small changes. The decision process for the EWMA chart is based on an exponentially decreasing (over time) function of prior measurements on the check standard while the decision process for the Shewhart chart is based on the current measurement only.  
Example of EWMA Chart 
In the EWMA control chart below, the red dots represent the measurements.
Control is exercised via the exponentially weighted moving average
(shown as the curved line) which, in this case, is approaching its
upper control limit.


Artifacts for process control must be stable and available
Case study: Resistivity 
The check standard artifacts for controlling the bias or longterm
variability of the process must be of the same type and geometry as
items that are measured in the workload. The artifacts must be stable
and available to the measurement process on a continuing basis. Usually,
one artifact is sufficient. It can be:


Topic covered in this section>  The topics covered in this section include: 