Step 4: Control (SDCA)©
Arthur M. Schneiderman
The control step assures that the metrics associated with the process remain stable. In this way, the fraction of output that fails to meet customer requirements can be predicted with a specified level of statistical confidence. With this assurance, the customer can effectively manage their inevitable non-conforming inputs through either 100% inspection (done by them or their supplier) or defect correction. The foundations of Control were laid by Walter Shewhart in the 1920's. Shewhart's basic premise was that through approximations of rigorous statistical formulas, he could make these techniques accessible to the first level operators thus eliminating the need for a cadre of costly statisticians.
Unfortunately, the resulting techniques often look like witchcraft to the average manager. Coupled with Shewart's (and his follower's) evasion of economic considerations (cost of a false alarm vs. cost of producing non-conforming output) these have undermined management support for process control. As a result, although there is much talk about SQC (watching the results metrics) and SPC (locking the process's critical nodes (process metrics)), I have seen little evidence of its widespread use outside of Japan.
Fortunately, today we have the ubiquitous PC. I have gone back and redone most of SQC using a math software package (MathCad) which eliminates the need for complex tables and formula while increasing the statistical rigor. I am in the process of adding economic considerations into the design of out-of-control action plans. It is my hope that the combination of real simplification (with increased rigor) and real cost/benefit analysis will be the keys to achieving the essential management support for this critical step.
Control is a prerequisite for process improvement. Without it, it is difficult or impossible to do experiments to identify root causes. Root cause analysis is central to both incremental improvement (Step 6) and process redesign (Step 7).
Last modified: August 13, 2006