2.2.2. How are bias and variability controlled?

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

2.2.2. How are bias and variability controlled?

Bias and variability are controlled by monitoring measurements on a check standard over time

Bias and long-term 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 measur ement sequences indicates possible:

Change or damage to the reference standards
Change or damage to the check standard artifact
Procedural change that vitiates the assumptions of the measurement process

A change in the variability of the measurements on the check standard can be due to one of many causes such as:

Loss of environmental controls
Change in handling techniques
Severe degradation in instrumentation.

The control procedure monitors the progress of measurements on the check standard over time and signals when a significant change occurs. There are two co ntrol 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 one-half 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 dashed 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 long-term 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 an d available to the measurement process on a continuing basis. Usually, one artifact is sufficient. It can be:

An individual item drawn at random from the workload
A specific item reserved by the laboratory for the purpose.

The topics covered in this section include: