Process variation is the main course of quality problems, whether in business (transactional) or production processes. The main task of statistical process improvement methods such as Statistical Process Control, Six Sigma and Measurement Systems Analysis and the Taguchi approach to Experimental Design is to control and reduce process variation.
Process variation and process precision are closely related. A process with little variation is said to be 'precise'. When studying process variation it is important to make a clear distinction between process accuracy and process precision. A process that is 'accurate' may not be 'precise' and a process that is 'precise' may not be 'accurate':

Not Precise 
Precise

Not Accurate



Accurate



Most processes are designed with controls that can be used to adjust the process mean, and hence increase the accuracy. Reducing the amount of process variation is usually more difficult.
