4. Variations
a. Variations in a process parameter may have several causes.
1) Random variations occur by chance. Present in virtually all processes, they are not correctable because they will not repeat themselves in the same manner. Excessively narrow control limits will result in many investigations of what are simply random fluctuations.
2) Implementation deviations occur because of human or mechanical failure to achieve results.
3) Measurement variations result from errors in the measurements of a results.
4) Model fluctuations can be caused by errors in the formulation of a decision model.
5) Prediction variances result from errors in forecasting data used in a decision model.