The depth of the analysis into the roots of the failure is the key to accurately unearthing all of the failure sources. Looking at machinery failures one finds that there are:
Physical Roots: The physical reasons why the parts failed.
Human Roots: The human errors of omission or commission that resulted in the physical roots.
Latent Roots (Management System Weaknesses): The deficiencies in the management systems or the management approaches that allow the human errors to continue unchecked.
The more detailed the analysis, the better we understand all the events and mechanisms that contribute as the roots of the problem. We generally think of dividing analyses into three categories in order of complexity and depth of investigation and they are:
Component Failure Analysis (CFA), which looks at the piece of the machine that failed, for example, a bearing or a gear, and determines that it resulted from a specific cause such as fatigue or overload or corrosion and that there were these x, y, and z influences.
Root Cause Investigation (RCI) is conducted in much greater depth than the CFA and goes substantially beyond the physical root of a problem to find the human errors involved. It stops at the major human causes and doesn't involve management system deficiencies. RCI's are generally confined to a single operating unit.
Root Cause Analyses (RCA) which includes everything the RCI covers plus the minor human error causes and, more importantly, the management system problems that allow the human errors and other system weaknesses to exist. An RCA can sometimes extend to sites other than the one involved in the original problem.
Although the cost increases as the analyses become more complex the benefit is that there is a much more complete recognition of the true origins. Using a CFA to solve the causes of a component failure answers why that specific part or machine failed and can be used to prevent similar future failures. Progressing to an RCI, we find the cost is five to ten times that of a CFA but the RCI adds a detailed understanding of the human errors contributing to the breakdown and can be used to eliminate groups of similar problems in the future. However conducting an RCA and correcting the major roots will eliminate huge classes of problems.
From "Understanding Why It Failed" by Neville W. Sachs, P.E. Sachs, Salvaterra & Associates, Syracuse, NY