T he expression “root cause” is somewhat misleading. Many people, includ- ing Six Sigma practitioners, use it without realizing its larger context. Asked what root cause means to them, some typical responses are: It’s what’s really happening; it’s the one thing that causes everything else; or it’s the switch that turns the light on. That is to say, they subscribe to the notion that there is one absolute thing that is the originator of the considered effects. This absolute origination is what people usually identify as the root cause. Books on quality or Six Sigma don’t always give clear descriptions of the concept of root cause. For example, although root cause analysis is a must-have tool, most Six Sigma or quality books explain only the tool without exploring the true meaning of root cause. Even the Juran Quality Handbook doesn’t have an entry for root cause. 1 One exception is the Six Sigma Black Belt Handbook, which provides a list of tenets that “underpin the belief that root causes can be fixed to improve processes,” citing that “they are must-be-accepted beliefs” for Six Sigma to reliably succeed. 2 Similar scenarios occur in Six Sigma training programs. I’ve been exposed to more than half a dozen Six Sigma programs offered by different vendors. Although most of them cover root cause analysis tools at various levels, many of them provide analysis of only the tools without attempting to explain what root cause really means. As a result, many practitioners might not even know when they discover the root causes. This is a major shortcoming of Six Sigma programs and should be corrected. Philosophy and the Root Cause Although not many quality or Six Sigma books bother attempting to explain the true meaning of root cause, the nature of the cause-effect relationship and root cause have been widely covered at the philosophical level outside the quality arena. The following is widely recognized in the philosophical world: The nature of a cause-effect relation- ship forms an infinite chain of causation. Any event is a cause and an effect at the same time. It is an effect of its upstream events and a cause of its downstream events. One can continue to R OOT CAUSE ANALYSIS Digging for the Root Cause DEFINING THE TERM IS THE FIRST STEP TO KNOWING WHEN YOU’VE FOUND THE ULTIMATE REASON FOR A DEFECT By Gary G. Jing, Entegris SIX SIGMA FORUM MAGAZINE I MAY 2008 I 19 Figure 1. Chain of Causation Effects Causes Five Whys ... Injury caused by fall Fall caused by wet surface Wet surface caused by leaky valve Leaky valve caused by seal failure Seal failure caused by poor maintenance