Causal tree logic trees

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. 

The earliest logic trees were based on engineering fault tree analysis methods. Today, companies use a number of variations or combinations of logic trees and call them by different names, such as Why Tree,< 9) Causal Tree,(20,21) Cause and Effect Logic Diagram (CELD),<22) and Multiple-Cause, Systems-Oriented Incident Investigation (MCS011).<23,24) tools have more similarities than differences. 

Examples of logic trees—fault, event, causal, and why—are discussed below in order of increasing rigor. Chapter 9 contains detailed information on developing logic trees. 

Causal Trees were developed in an effort to use the principles of deductive logic found in Fault Tree but make it more user-friendly. Originally, private companies developed the Causal Tree Method (CTM) for safety, process safety, and environmental incident investigations applications. Rhone-Poulenc, for example, was an early user.<20.21) Multiple-Cause Systems Oriented Incident Investigation (MCSOfl) is another name for the CTM. At this time, most companies use simplified versions of fault trees for complex incident investigations. 

Many deductive investigation techniques use logic tree diagrams. A partial list of these methods includes fault tree analysis (FTA), causal tree... 

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. 

The Oval A conditional event or a conditional input into the logic tree that further defines the state of the system which must exist in order for the fault sequence to occur. It may place a restriction on event occurrence based on the occurrence of other events on the causal chain. 

MORT is an analysis technique for identifying SR oversights, errors, and/or omissions that led to the occurrence of a mishap. MORT is primarily a reactive analysis tool for accident/mishap investigation, but it can also be used for the proactive evaluation and control of hazards. MORT analysis is used to trace out and identify all of the causal factors leading to a mishap or UE. MORT utilizes the logic tree structure and rules of FTA, with the incorporation of... 

The investigation-system style we prefer is one that uses free-flow logic that documents the relationships among the facts of an event. These are usually referred to as causal-tree diagrams. We prefer strong logic theory and a... 

A number of deductive techniques require that the investigation team develop a tree. This is accomplished by reasoning to organize causal factors into a diagram (tree) and define their interrelationship. These logic... 

Fault Tree Analysis (FTA) is a well known and widely used safety tool, implementing a deductive, top down approach. It starts with a top level hazard, which has to be known in advance and "works the way down" through all causal factors of this hazard, combined with Boolean Logic (mainly AND and OR gates). It can consider hardware, software and human errors and identifies both single and multiple points of failure. Both a quantitative and qualitative analysis is possible. 

A Fault Tree Analysis (FTA) is a top-down, deductive logic model that traces the failure pathways for a predetermined, undesirable condition or event, called the TOP Event. An FTA can be carried out either quantitatively or subjectively. The FTA generates a fault tree (a symbolic logic model) entering failure probabilities for the combinations of equipment failures and human errors that can result in the accident. Each immediate causal factor is examined to determine its subordinate causal factors until the root causal factors are identified. 

Each of the Core Hazards has been analysed in turn, attempting to determine all possible causalities of the Core Hazard, and the logical relationships between identified causahties leading to the materiahsation of the Core Hazard. As result of this, a fault tree has been developed for each Core Hazard. 

For fault tree analysis, the starting point is to specify an undesirable serious situation, called the top effect, and then to consider all possible causes that could produce it. For example, the specified top effect could be the over-pressurization of a chemical reactor. Possible causes could include a reduction or loss of coolant, excess catalyst, an ineffective pressure control loop, etc. Each possible cause is analyzed further to determine why it occurred. For example, the pressure control loop problem could be to the result of a sensor or control valve malfunction. Thus, FTA is a top-down approach that generates a tree of causal relations, starting with the specified top event and working backward. Standard logic concepts, such as AND and OR, are used in the logic diagrams. 

Fault Tree Analysis employs an analytical tree to display the results of an analysis (Suokas and Rouhiainen, 1993). It starts with the top event (injury or damage). The analysis proceeds backwards in order to identify all events and conditions that have caused the injury or damage. Logical relations (necessary and/or sufficient conditions) are estabhshed. Fault-tree analysis is not an accident model per se and gives limited support in the identification of causal factors. 

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