Predefined trees causal factors

In general, the companies surveyed use one of two main methodologies to determine root causes. The first involves timeline construction followed by logic tree development. The second involves timeline construction, identification of causal factors, followed by the use of predefined trees or checklists. These two approaches are discussed in detail in Chapter 9. 

Once the causal factors have been identified, the factors are analyzed using a root cause analysis tool, such as 5-AVhys or predefined trees. See Chapter 9 for a more detailed discussion of Barrier Analysis (sometimes called hazard-barrier-target analysis or HBTA) and Change Analysis (also referred to as Change Evaluation/Analysis or CE/A). In essence, these tools act as a filter to limit the number of factors, which are subjected to further analysis to determine root causes. 

Timeline construction, identification of causal factors, and predefined trees or checklists. 

The causal factors need to be examined further to determine why those factors existed. The investigation team may use a predefined tree to examine each causal factor individually. The first causal factor is analyzed starting at the top of the tree, and then working down all of the branches as far as the facts permit. When an appropriate subcategory on one of the branches is identified, it is recorded as a root cause. The remaining branches are checked as one causal factor may have multiple root causes. The procedure is then repeated for each causal factor in turn. 

After the predefined tree has been used, a final generic cause test should be applied. The plant operating history, especially previous incidents, is considered to indicate if other generic management system problems exist. For example, repetitive failures may indicate generic causes that would not be apparent by only investigating the current incident. It is also an opportunity for a final overall review of the investigation to focus on the big picture, not just individual facts or causal factors. The team should ask, Are there any other causes that anyone has in mind that have not been included ... 

Once the actual incident scenario is understood and its multiple causal factors identified, this information may be used to determine the incident s root causes. One means of performing root cause analysis involves the use of ready-made, predefined trees. A predefined tree provides a systematic approach for analyzing and selecting the relevant elements of the incident scenario. It is a deductive approach, looking backward in time to examine preceding events necessary to produce the specified incident. 

Unlike the procedure followed in developing logic trees, the investigation team does not construct the tree. Rather they apply each causal factor to each branch of the predefined tree in turn, and those branches that are not relevant to the incident are discarded. This prescriptive approach offers consistency and repeatability by presenting different investigators with the same standard set of possible root causes for each incident. 

While the use of predefined trees does not directly challenge the investigation team to think laterally of other possible causes, many predefined trees present a wide range of causal factors, some of which the team may not have otherwise considered. It is therefore possible, but unlikely, that the incident could involve a novel root cause that was not previously experienced by those who developed the predefined tree. The addition of a final test based on another tool, such as brainstorming, can overcome this apparent weakness. 

The first causal factor is then analyzed starting at the top of the predefined tree and working down the branches as far as the facts... 

Each of the causal factors can now he analyzed for its specific root causes using a predefined tree, as shown in Figure 9-3 f. 

The following is an analysis of one of these causal factors contractor operator (CO) falls asleep. The basic technique works with any of the predefined trees commonly used within the process industry. However, for the purposes of this example, a proprietary tool C) has been selected, and therefore the structure of the tree and the terminology used is specific to that tree. 

In this case, the causal factor (contract operator falls asleep) is identified as a Human Performance Difficulty (one of the four major problem categories at the top of the tree), and the other three categories are discarded. (Different predefined trees use different terminology and structure, but generally cover similar choices.)... 

The root causes identified by applying the causal factors to a predefined tree should be subjected to a management system test to ensure that they are management system failures. Some predefined trees are quite detailed, while others do not necessarily fully reach the root cause level. The system test essentially applies the 5-Whys tool to each cause identified at the end of the relevant branches of the predefined tree. 

Predefined trees are a convenient means of identifying root causes. Providing all of the causal factors have been determined, use of a comprehensive predefined tree should ensure that most, if not all, root causes are identified. Several quality assurance tests should help identify any remaining root causes. 

A timeline or sequence diagram is first developed, and then causal factors identified. Care should be taken to ensure that the checklist is not used too early. Be sure to determine what happened and how it happened before determining why it happened. Otherwise, the team will think that they have identified the right root cause(s), when in reality only one or two of several multiple causes have been determined. The causal factors are then applied one at a time to each page of the checklist(s) to identify relevant root causes. Those pages that are not relevant to the particular incident of interest are discarded. Similar quality assurance checks should be applied as those described for predefined trees. 

The article (Rooney Vanden Heuvel, 2004) uses the term Root Cause Map as an equivalent to the term predefined tree, when the fourth step of the method B of root cause analysis is discussed. According to the article, root cause identification involves the use of a decision diagram called the Root Cause Map to identify the underlying reason or reasons for each causal factor . Fig. 3 shows a small part of a specific Root Cause Map that is reproduced from (CCPS, 2003). It is a subtree of larger decision tree. Displayed part helps the investigator identify detail causes in the SMS, namely, in a fraction of its component (iii) operation control. 

Suppose that a causal factor chart is developed (see example in Fig. 1). However, there is no warranty, that each of the specific chains of underlying causes from the causal factor chart wiU be coimected to a corresponding generic chain of underlying causes from the predefined tree. Some of causal factors may have no root causes. 

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