Root cause analysis (RCA) is a class of problem solving methods aimed at identifying the root causes of problems or events.

Root Cause Analysis is any structured approach to identifying the factors that resulted in the nature, the magnitude, the location, and the timing of the harmful outcomes (consequences) of one or more past events in order to identify what behaviors, actions, inactions, or conditions need to be changed to prevent recurrence of similar harmful outcomes and to identify the lessons to be learned to promote the achievement of better consequences.

The practice of RCA is predicated on the belief that problems are best solved by attempting to address, correct or eliminate root causes, as opposed to merely addressing the immediately obvious symptoms. By directing corrective measures at root causes, it is more probable that problem recurrence will be prevented. However, it is recognized that complete prevention of recurrence by one corrective action is not always possible.

Nevertheless, in the U.S. nuclear power industry the NRC requires that "In the case of significant conditions adverse to quality, the measures shall assure that the cause of the condition is determined and corrective action taken to prevent repetition." [10CFR50, Appendix B, Criterion XVI, Sentence 2)] In practice more than one "cause" is allowed and more than one corrective action is not forbidden.

A root cause is rarely an initiating cause of a causal chain which leads to an outcome or effect of interest. Commonly, root cause is misused to describe the depth in the causal chain where an intervention could reasonably be implemented to change performance and prevent an undesirable outcome.

In plain English a "root cause" is a "cause" (harmful factor) that is "root" (deep, basic, fundamental, underlying or the like).

The term root cause has been used in professional journals as early as 1905, but the lack of a widely accepted definition after all this time indicates that there are significantly different interpretations of exactly what constitutes a root cause.

Many governmental investigation agencies avoid the term or use it informally. Others use it in a stilted formalistic or bureaucratic manner. The National Transportation Safety Board, with an excellent investigative reputation, uses the term "probable cause." (In law enforcement "probable cause" has a very different meaning.)

A common view of "root cause" follows.