Logic tree

Figure 64.1 illustrates the logic tree to follow for an OSHA-mandated investigation. While it is similar to other non-mandated investigations, there are distinct differences. 

Figure 64.1 Logic tree for an OSHA-mandated investigation...

This chapter addresses methods and tools used successfully to identify multiple root causes. Process safety incidents are usually the result of more than one root cause. This chapter provides a structured approach for determining root causes. It details some powerful, widely used tools and techniques available to incident investigation teams including timelines, logic trees, predefined trees, checklists, and fact/hypothesis. Examples are included to demonstrate how they apply to the types of incidents readers are likely to encounter. 

The disciplines of engineering and quality control have long recognized the principles of root cause analysis. Some process safety tools for root cause analysis have been borrowed from these disciplines. For example, fault tree analysis was developed as an engineering tool, but its logic tree structure has been adapted to meet process safety requirements. 

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. 

Logic trees are committee-based investigation toois that use a muitipie cause, system-oriented approach to determine root causes integrated with process safety management program. Exampies fauit tree, event tree, causai tree, and why tree. 

Checklists may also be used to supplement other tools for example, checklists on human factors may be used in conjunction with logic trees. Similarly, checklists may be used in combination with structured brainstorming tools such as What If/Checklist and Hazard and Operability (HAZOP) Analysis.(P It is also a good practice to apply a tool like the 5-Whys to the root causes identified from the checklist to verify whether they are truly root causes. 

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. 

There are five main strengths to a logic tree approach. 

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. 

Another type of logic tree, the event tree, is an inductive technique. Event Tree Analysis (ETA) also provides a structured method to aid in understanding and determining the causes of an incident.(i) While the fault tree starts at the undesired event and works backward to identify root causes, the event tree looks forward to display the progression of various combinations of equipment failures and human errors that result in the incident graphically. 

Timeline construction followed by logic tree development. 

In addition, the team should also update the in-progress analysis of the occurrence. For example, update the fact list, logic tree, or fact hypothesis matrix. Finally, the information from the interview must he communicated to the remainder of the investigation team. 

Figure 9-1, the two flowcharts describing root cause determinations using Methods A and B, presents general frameworks for root cause determination. Method A focuses on the logic tree method using a simplified fault tree approach. Method B focuses on the predefined tree method. 

AVhile some methods use checklists as the logic analysis step, an understanding of the logic tree approach is still helpful because checklists are developed from logic trees. Checklists are especially helpful when related to human factor issues. A sample checklist is included in Chapter 6 as Figure 6-6 (page 94). 

The diagram can assist the overall methodology to identify multiple causes, when used in conjunction with logic trees or predefined trees. 

Root Cause Determination Using Logic Trees—Method A... 

FfGURE 9-9. Flowchart for root cause determination using logic trees. 

Next the team develops a chronology of events based on the available known times and sequences. This document is usually referred to as a timeline. (See the detailed description of timeline development in Section 9.3.) Unconfirmed assumptions regarding chronology should be clearly identified as unconfirmed, and action should be initiated to verify assumptions. Many investigators use relatively simple timelines (instead of sequence diagrams) with the logic tree methods because the logic tree itself shows the interactions of events and conditions. 

After the initial facts have been listed and the initial timeline developed, the logic tree diagram can be constructed. The tree diagram is a dynamic document it continues to expand and may even rearrange as additional information becomes available or when new information changes the understanding of the original facts. 

In the opening segment the facilitator should discuss the importance of and methods for choosing the top event and any preestablished and existing boundaries of the investigation. If multiple events are involved, it is best to start with the last event in the time sequence. It may be appropriate, depending on the nature of the occurrence, to formally review the rules and symbols used in logic tree or fault tree development or whichever other formal method will be used. 

At this point, the logic tree structure is examined to ensure the tree is logically consistent and compatible with the known facts. In some instances, there may be inconsistencies and application of the fact/hypoth-esis matrix will be appropriate. This powerful tool is described in more detail later in this chapter. Inconsistencies found at this point require further tree development or rearrangement. 

Once the logic tree structure appears to be consistent, the first of three quality assurance tests is applied by examining the overall logic tree structure for completeness. The logic in each branch of the tree should be tested to determine if it is necessary and sufficient. (Details and tips for testing the logic are discussed in Section 9.6.2.) If the tree appears to be complete, the next quality assurance test is initiated. If the tree is incomplete, then the fact or logic problem is identified and the entire process is repeated. This is called an iterative loop. ... 

After the tree is developed, and before moving on to the recommendations and deliberations, the team should ask, Are there any other causes that anyone had in mind at the beginning of this meeting that are not included in the tree If additional causes are identified, the team adds them to the tree if there is logic to support them. Some team members may have specific concerns that the logic tree has not adequately resolved. This is the point at which remaining issues are surfaced and addressed. 

The next item in the loop is a decision point for possibly introducing the use of inductive reasoning methods into the deliberations. If the deductive process continues to indicate progress, then additional facts are procured or the logic tree is restructured. For example, one witness stated a particular valve was open, yet the post-incident inspection found it to be closed. The team must be careful to ensure that the valve is closed because of the actions taken prior to the incident, and not as a result of post-event response activities. The position of this particular valve may be a critical item in determining which of two scenarios is the more probable case. The incident investigation team would then initiate a short-term action item to conduct a mini-investigation to resolve this question. 

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

To put it simply, a logic tree is developed by repeatedly asking Why and organizing the results of the answers. 

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