Two Faces of SPC

Closed-loop feedback control uses feedback to control system deviations from the desired state. If the deviation exceeds some pre-defined control level, a controlled should provide a control signal having an effect on the process outputs.

There are three possible versions of feedback control in industry:

1. ‘Voice of Customer’ Control based on Customer’s Complaints (Worst Case of feedback control)
2. ‘Voice of Inspection’ Control based on ‘Postmortem’ Non-Conforming Detection during product testing/inspection (Medium Case of feedback control)
3. Voice of Process’ Control based on Results of Process Monitoring (Best Case of feedback control) – SPC.

As the old Roman God Janus, SPC also has two faces:

• The right side deals with product quality evaluation – Statistical Product Characterization (Monitoring of Critical Product Characteristics)
• The left side of the face looks after the process behavior – Statistical Process Control (Monitoring of Critical Process Parameters)

Unfortunately, there is a lot of misunderstanding concerning a difference between these two SPC versions. Most SPC activities in industry represent kind of Statistical Product Characterization, although all involved parties call it Statistical Process Control.

This paper presents and explains the difference between two SPC versions and contains the case studies demonstrating them.