One to change the lightbulb and five to perform a Fault Tree Analysis (FTA). A fault tree is a diagram that illustrates the relationship between failures at component and system levels. Engineers use them to understand the mechanisms or logic that lead from component malfunctions to system breakdowns and to identify components that are critical to system reliability. They are useful in optimizing designs, demonstrating compliance with safety requirements and as diagnostic tools when things go wrong. There are some simple examples of fault trees for ‘no light in room’ and ‘missing the bus’ amongst others available from Visual Paradigm Online. All of these examples illustrate qualitative relationships but we can also establish quantitative relationships using the rate of occurrence of each initiating event to arrive at a probability of failure (PoF) for the system. There is an example for an indicator light in an automobile in a 2016 paper by Nabarun Das and William Taylor (see figure 2 in the paper). An equivalent in biology are Adverse Outcome Pathways (AOPs) that identify the relationship between a molecular initiating event and a toxic effect through a series of key events. For instance, microplastics causing altered gene expression and oxidative damage leading to altered fatty acid metabolism, stress response and altered cellular division resulting ultimately in population decline in aquatic species as shown in the graphic from a paper by Tamara Galloway and Ceri Lewis also published in 2016. Most AOPs are qualitative; however, quantitative Adverse Outcome Pathways (qAOPs) are starting to be developed as tools for quantitative risk assessment of chemicals. Biologists and engineers are not using the same words, actually they are using entirely different vocabularies; nevertheless they are talking about the same methodologies. An AOP network and an FTA are essentially the same concept and a probabilistic fault tree analysis is a quantitative adverse outcome pathway. However, it seems unlikely that either biologists or engineers will adopt the language used by the other so they will be reliant on a few foolhardy interlocutors prepared to cross the discipline boundaries and highlight the opportunities for cross-fertilization of ideas and solutions.
Das N, Taylor W. Quantified fault tree techniques for calculating hardware fault metrics according to ISO 26262. In2016 IEEE Symposium on Product Compliance Engineering (ISPCE), pp. 1-8. IEEE, 2016. Also available at https://incompliancemag.com/article/quantified-fault-tree-techniques-for-calculating-hardware-fault-metrics-according-to-iso-26262/
Galloway TS, Lewis CN. Marine microplastics spell big problems for future generations. Proceedings of the national academy of sciences. 113(9):2331-3, 2016.