In a set of various causes to a non conformance, the one cause if resolved will not recur again or move to the other process is called the "Primary Root Cause". For example while analyzing why a person is late to work, the root cause could be as simple as forgetting to replace the battery of an alarm clock. It is that cause beyond which you cannot ask the "Why" and this cause will lead to recommendation of solution. Also I could reflect to one root cause that my quality assurance team conducted on the associate entering the bill data and the supporting as multiple bills in the system. The root cause here was the associate did not bother to look at "Bill to" on the bills and supporting. The quality assurance team further educated the Accounts payable team to read the details on the bill payable and introduced a pop up for every mandatory field to be entered in the system which was an error proofing solution.