Process Stability and Capability the "forgotten" limitations to Process Improvement

Recently, I have been reminded that many manufacturers undertake process improvement initiatives before they have an adequate understanding of their underlying process stability and whether or not they have a capable process. Often, in our rush to drive improvement we overlook the basics in process design and capability.

This article is to encourage those that are about to embark on process improvement initiatives or whose improvement efforts have stalled - to take a step back and ensure they are evaluating process stability (influence of only common causes) and capability (Cp / Pp) as they work to drive improvement.

First, it is imperative to ensure that processes are stable, that regardless of the range of results the underlying variation is attributable to only common causes - things like mix time or moisture, as examples - and the process results are consistent and repeatable - again regardless of the range of results. Deming suggests that something like 94% of issues are caused by common causes - so the quest for stability and definition of common causes within the process can often provide understanding or trigger work to reduce variation and improve performance.

Next, in many production lines today - multiple processes can be arrayed without regard for their influence on overall system (line) capability. It is important that process improvement teams evaluate component processes for their individual stability and capability - and then appropriately define how those component processes interact to determine system/line capability. Watch out for serial configurations that compound variability or parallel configurations based on equipment of different capacities or manufacturers.

Many times, when finished goods specifications are tighter then process capability - a system can be forced into "compliance" through high levels of inspection and rework but the process/system remains "incapable". It is far more productive to ensure that capability is the focus vs. control and compliance. For example, in a production line where there is a portion forming (slicing, depositing, etc.) step early in the system/line - followed by a weight control (check-weighing, inspection, etc.) step - it is possible to attain a high level of compliance by allowing a high percentage of give-away (overweights) and rework (underweights). Meaning the customer can receive products that are above minimum weights/portions with a high level of confidence - but that system/line still has significant room for improvement.

Assuming a stable process, the overlay of specification and capability is quickly summarized using Process Capability calculations. The understanding of Process Capability relative to specifications can help shift the focus from compliance to improvement and is extremely valuable when evaluating investments that can drive capability vs. improve compliance. Using the example above - an investment that improves capability in portioning will likely payback much better than an investment in higher resolution weight control - though both can contribute to higher levels of confidence for the customer. Capability is an underlying concept of Six Sigma.

I'd love to speak with you regarding capability, stability and reliability for your Operations; [email protected]. Process capability can have significant influence on Operations profitability - particularly in contract manufacturing and other applications - let's discuss how it may be impacting you and what you can do to improve.