Process improvement is based on optimising the performance of the organisation (system) as a whole by carefully developing (improving) each and every activity (process) in harmony. This is the basis of both Customer Delight and a reduction in the Cost of Quality.
Having identified the organisation as a system made up of many interacting and interdependent processes, we then need to study each individual process to discover how well it is working and what it could (and should) be capable of in the future.
There is, for example, no point in promising a 24-hour turnround if the process is only capable of a 48-hour one. And there is little point setting a target of 100 units per day if the machine can only process 50. Attempting to force the process in either case will result in cutting corners, which will upset customers (and staff) and increase the Cost of Quality.
This is where Statistical Process Control (SPC) comes in.