Process control charts, whether being used to chart variable or attribute data, help us to understand the current operating status of a process. A control chart is a pictorial representation of process behaviour, and it is generally considered more valuable than a thousand words!
Where the process variability and average remain constant, variation between individual samples would be caused by chance alone. Plots would seldom fall outside the control limits and there would be no obvious trends or patterns within the data.
Careful analysis and interpretation of control charts allows us to identify signals that a process is not in a state of statistical control. By responding appropriately to these signals early enough we can nip problems in the bud, thereby reducing unnecessary scrap or rework.