When tackling process improvement, deming found that the the variation was critical?

The knowledge of variation was critical.

Most important is understanding what is below.

It seems that most business executives were not trained on understanding processes and variation. They study how to manage people and money, but not how to listen to a process through data, and use that data to make improvements. Because many are not familiar with Dr. W. Edwards Deming’s enlightened insights on data and variation, they are unaware of the importance of process data and that different types of variation exist –and that those different types of variation require different types of responses. Deming also said, "How would they know?" If no one ever taught them (even worse if they were taught approaches that seem to work –even though in reality they sometimes do more harm than good), indeed, how would they know?

The point is this: when the wrong data is used or different types of variation go unrecognized, undiagnosed, or are confused, the resulting decisions and actions tend to increase costs, reduce quality, reduce productivity, and foster frustration throughout the organization.

Simply put, Dr. Deming emphasized in his writings, that business leaders have typically been taught to treat everything they don’t like as having a "special cause" reason as to why it happened, and thus want to investigate what one thing or person was responsible for causing the "aberration". People in general, seem to be wired and trained to go looking for THE reason that something bad or good happened. This problematic approach is often reinforced, because we can usually find "something unusual" associated with the thing we are investigating. Unfortunately, this "something unusual" is rarely the cause of the problem