The first objective in statistical process control is to eliminate assignable causes of variation in the process variable and then get the process in control. The next step is to reduce variation and get the measurements on the process variable within specification limits, the limits within which the measurements on usable items or services must fall.

Assignable Causes

Specific, identifiable factors leading to variations in a process or system, as opposed to random variations.

Variation

The degree to which data points in a statistical distribution or dataset differ from each other and from the mean of the set.

Specification Limits

Pre-determined upper and lower bounds within which a product or process's performance is considered acceptable.