pg_0023

Process Improvement

5.2.

Assumptions

5.2.2.

Is the process stable.

Plan to

examine

process

stability as

part of your

experiment

Experimental runs should have control runs that are made at the

`standard' process setpoints, or at least at some standard operating

recipe. The experiment should start and end with such runs. A plot of

the outcomes of these control runs will indicate if the underlying process

itself has drifted or shifted during the experiment.

It is desirable to experiment on a stable process. However, if this cannot

be achieved, then the process instability must be accounted for in the

analysis of the experiment. For example, if the mean is shifting with

time (or experimental trial run), then it will be necessary to include a

trend term in the experimental model (i.e., include a time variable or a

run number variable).

5.2.2. Is the process stable.

http://www.itl.nist.gov/div898/handbook/pri/section2/pri22.htm [5/7/2002 4:01:43 PM]