6. Short-term / Long-term: take all process dispersion factors into account

Manufacturing processes disperse influencing quantities due to variations. Their dispersions are assessed on the basis of entities sampled from time to time (sampling frequency). To be representative of reality, these samples must be taken in such a way that the influencing variables have actually had the opportunity to express their own variations. For example, samples taken from the same batch of raw material (assuming that the raw material is influential) cannot be used to estimate the impact of variation due to the raw material. What is true for the raw material is also true for any other influential variable. Each of them must therefore have had the opportunity to vary over the course of the samplings if the estimated dispersion of the process is to be representative of its reality.

For capability studies, the notions of "short-term" and "long-term" capability...