Control Charts With Non-normal Distributions
Jacobs (1990) discussed control charts with non-normal
distributions. Some ways that these distributions arise are:
- restraining a variable at a fixed limit, e.g., the process
temperature can never exceed some critical setpoint;
- the material has a physical limit such as melting or boiling point;
- zero is a natural limit, e.g., no negative concentration of impurity;
- variables may relate non-linearly, e.g., reaction rate versus
temperature.
Once the distribution is known, the lines for control
limits can be derived in a manner analogous to that for the usual
Shewhart chart. These lines will not be equidistant from the mean.
Jacobs, D,C., Watch out for non-normal distributions, Chem. Eng. Prog. 86: 19-27 (1990)