Fault Tree Analysis Example

The FTA diagram sample "Fault tree analysis - Insulin delivery system" was redesigned from the illustration of "CMSI 641: Introduction to Software Engineering. Design of Critical Systems. B.J. Johnson. 2005. Loyola Marymount University".
"Another way of assessing hazards is using fault tree analysis. In this process, each of the identified hazards is covered by a detailed analysis to find out what might cause it. Either inductive or deductive reasoning is applied. In the case of software hazards, the usual focus is to determine faults that will cause the system to fail to deliver a system service, such as a monitoring system. A "fault tree" is constructed to link all the possible situations together, to help identify the interrelationships of the failures, which modules may cause them, and what "trickle-down effects" there might be. Here is an example of a fault tree, as applied to the Insulin delivery system from Sommerville...
Note that this tree is only partially complete, since only the potential software faults are shown on the diagram. The potential failures involving hardware, such as low battery, blood monitor or sensor failure, patient over-exertion or inattention, or medical staff failure are noticeable by their absence.
The fault tree and safety specification processes are two ways of helping with system risk assessment tasks. Once the risks are identified, there are other assessments that need to take place. First, the likelihood of the risk occurrance must be assessed. This is often quantifiable, so numbers may be assigned based on things like MTBF, latency effects, and other known entities. There may be other non-quantifiable contributors to the risk likelihood, however, such that these must be assessed and estimated by experts in the domain. (Don't short-change this process when dealing with critical systems!) Finally, the risk assessment must include the severity of the risk, meaning an estimation of the cost to the project in the event the risk item actually does occur. "Cost to the project" means all associated costs, including schedule delays, human injury, damage to hardware, corruption of data, and so on."
[myweb.lmu.edu/ bjohnson/ cmsi641web/ week15-2.html]
The FTA diagram example "Fault tree analysis - Insulin delivery system" was created using the ConceptDraw PRO diagramming and vector drawing software extended with the Fault Tree Analysis Diagrams solution from the Engineering area of ConceptDraw Solution Park.

The vector stencils library "Fault tree analysis diagrams" contains 12 symbols for drawing Fault Tree Analysis (FTA) diagrams.
"Fault tree analysis (FTA) is a top down, deductive failure analysis in which an undesired state of a system is analyzed using Boolean logic to combine a series of lower-level events. This analysis method is mainly used in the fields of safety engineering and reliability engineering to understand how systems can fail, to identify the best ways to reduce risk or to determine (or get a feeling for) event rates of a safety accident or a particular system level (functional) failure. FTA is used in the aerospace, nuclear power, chemical and process, pharmaceutical, petrochemical and other high-hazard industries; but is also used in fields as diverse as risk factor identification relating to social service system failure.
In aerospace, the more general term "system Failure Condition" is used for the "undesired state" / Top event of the fault tree. These conditions are classified by the severity of their effects. The most severe conditions require the most extensive fault tree analysis. These "system Failure Conditions" and their classification are often previously determined in the functional Hazard analysis." [Fault tree analysis. Wikipedia]
The shapes example "Fault tree analysis diagrams" was created using the ConceptDraw PRO diagramming and vector drawing software extended with the Fault Tree Analysis Diagrams solution from the Engineering area of ConceptDraw Solution Park.

"Failure analysis is the process of collecting and analyzing data to determine the cause of a failure. It is an important discipline in many branches of manufacturing industry, such as the electronics industry, where it is a vital tool used in the development of new products and for the improvement of existing products. There are many companies which provide services to find the cause of failure in products, devices and in post disaster situations. The failure analysis process relies on collecting failed components for subsequent examination of the cause or causes of failure using a wide array of methods, especially microscopy and spectroscopy. The NDT or nondestructive testing methods (such as Industrial computed tomography scanning) are valuable because the failed products are unaffected by analysis, so inspection always starts using these methods." [Failure analysis. Wikipedia]
The example "Audit failure - Fault tree analysis diagram" was created using the ConceptDraw PRO diagramming and vector drawing software extended with the Fault Tree Analysis Diagrams solution from the Engineering area of ConceptDraw Solution Park.

This example was redesigned from the Wikimedia Commons file: Example of High Level Fault Tree.jpg. [commons.wikimedia.org/ wiki/ File:Example_ of_ High_ Level_ Fault_ Tree.jpg]
"Risk assessment is the determination of quantitative or qualitative value of risk related to a concrete situation and a recognized threat (also called hazard). Quantitative risk assessment requires calculations of two components of risk (R):, the magnitude of the potential loss (L), and the probability (p) that the loss will occur. Acceptable risk is a risk that is understood and tolerated usually because the cost or difficulty of implementing an effective countermeasure for the associated vulnerability exceeds the expectation of loss.
In all types of engineering of complex systems sophisticated risk assessments are often made within Safety engineering and Reliability engineering when it concerns threats to life, environment or machine functioning. The nuclear, aerospace, oil, rail and military industries have a long history of dealing with risk assessment. Also, medical, hospital, social service and food industries control risks and perform risk assessments on a continual basis. Methods for assessment of risk may differ between industries and whether it pertains to general financial decisions or environmental, ecological, or public health risk assessment." [Risk assessment. Wikipedia]
The FTA diagram example "High level fault tree" was created using the ConceptDraw PRO diagramming and vector drawing software extended with the Fault Tree Analysis Diagrams solution from the Engineering area of ConceptDraw Solution Park.

This FTA diagram example was redesigned from the Wikipedia Commons file: Example Code Fault Tree.jpg. [commons.wikimedia.org/ wiki/ File:Example_ Code_ Fault_ Tree.jpg]
"Use of Software Fault Tree Analysis.
Any SFTA must be preceded by a hazard analysis of the entire system. The information in the hazard analysis identifies those undesired events in the system that can cause serious consequences. It should be noted that in comple x systems not all hazards can be predetermined.
In this respect the technique does not claim to produce consistent results irrespective of the analyst. It is dependent on the judgment of the individual as to when to stop the process and which hazards to analyze.
The SFTA can be used at different stages of the software life cycle, beginning at the software requirements phase. At that early stage, only the bare minimum of the software will be defined, though the system will usually have a preliminary design. Only a very high-level SFTA will be able to be performed. However, as the software design progresses, and the code is developed, the SFTA can be “fleshed out” with the new information. High-level blocks can be broken down into specific components (modules, classes, methods, etc.)." [NASA Software Safety Guidebook. hq.nasa.gov/ office/ codeq/ doctree/ 871913.pdf]
The FTA diagram example "Code fault tree" was created using the ConceptDraw PRO diagramming and vector drawing software extended with the Fault Tree Analysis Diagrams solution from the Engineering area of ConceptDraw Solution Park.

The accident analysis diagram example "Accident analytic tree" was redesigned from the picture 7-14 from the "DOE Workbook. Conducting Accident Investigations. Revision 2."
"Analytic tree analyses are well defined, useful methods that graphically depict, from beginning to end, the events and conditions preceding and immediately following an accident. An analytic tree is a means of organizing information that helps the investigator conduct a deductive analysis of any system (human, equipment, or environmental) to determine critical paths of success and failure. Results from this analysis identify the details and interrelationships that must be considered to prevent the oversights, errors, and omissions that lead to failures. In accident investigations, this type of analysis can consist of both failure paths and success paths, and can lead to neutral, negative, or positive conclusions regarding accident severity.
TIP.
An analytic tree enables the user to:
(1) Systematically identify the possible paths from events to outcome.
(2) Display a graphical record of the analytical process.
(3) Identify management system weaknesses and strengths."
[homer.ornl.gov/ sesa/ corporatesafety/ aip/ docs/ workbook/ Rev2/ chpt7/ chapt7.htm]
The FTA diagram example "Accident analytic tree" was created using the ConceptDraw PRO diagramming and vector drawing software extended with the Fault Tree Analysis Diagrams solution from the Engineering area of ConceptDraw Solution Park.

The FTA diagram example "Hazard analysis" was redesigned from the illustration of the article "Hazard Analysis Methodologies" from the Occupational Safety & Health Administration (OSHA) website.
"A fault tree analysis is a quantitative assessment of all of the undesirable outcomes, such as a toxic gas release or explosion, which could result from a specific initiating event. It begins with a graphic representation (using logic symbols) of all possible sequences of events that could result in an incident. The resulting diagram looks like a tree with many branches — each branch listing the sequential events (failures) for different independent paths to the top event. Probabilities (using failure rate data) are assigned to each event and then used to calculate the probability of occurrence of the undesired event. ...
This technique is particularly useful in evaluating the effect of alternative actions on reducing the probability of occurrence of the undesired event."
[osha.gov/ SLTC/ etools/ safetyhealth/ mod4_ tools_ methodologies.html]
The FTA diagram example "Hazard analysis" was created using the ConceptDraw PRO diagramming and vector drawing software extended with the Fault Tree Analysis Diagrams solution from the Engineering area of ConceptDraw Solution Park.

"Subjective performance evaluation allows the use of a subtler, more balanced assessment of employee performance, and is typically used for more complex jobs where comprehensive objective measures are difficult to specify and/ or measure. Whilst often the only feasible method, the attendant problems with subjective performance evaluation have resulted in a variety of incentive structures and supervisory schemes. One problem, for example, is that supervisors may under-report performance in order to save on wages, if they are in some way residual claimants, or perhaps rewarded on the basis of cost savings. This tendency is of course to some extent offset by the danger of retaliation and/ or demotivation of the employee, if the supervisor is responsible for that employee’s output. ...
Another problem relates to what is known as the "compression of ratings". Two related influences—centrality bias, and leniency bias—have been documented ... The former results from supervisors being reluctant to distinguish critically between workers (perhaps for fear of destroying team spirit), while the latter derives from supervisors being averse to offering poor ratings to subordinates, especially where these ratings are used to determine pay, not least because bad evaluations may be demotivating rather than motivating. However, these biases introduce noise into the relationship between pay and effort, reducing the incentive effect of performance-related pay. ... this is the reason for the common separation of evaluations and pay, with evaluations primarily used to allocate training.
Finally, while the problem of compression of ratings originates on the supervisor-side, related effects occur when workers actively attempt to influence the appraisals supervisors give, either by influencing the performance information going to the supervisor: multitasking (focussing on the more visibly productive activities...), or by working “too hard” to signal worker quality or create a good impression...; or by influencing the evaluation of it, e.g., by "currying influence"... or by outright bribery..." [Principal–agent problem. Wikipedia]
The example "Person demotivated by evaluation - Fault tree analysis diagram" was created using the ConceptDraw PRO diagramming and vector drawing software extended with the Fault Tree Analysis Diagrams solution from the Engineering area of ConceptDraw Solution Park.

Fault tree analysis diagrams solution extends ConceptDraw PRO software with templates, samples and library of vector stencils for drawing the FTA diagrams.