Why-Because Analysis
“Why-Because Analysis” (WBA) is a rigorous method for analysing unwanted system behaviour such as accidents. WBA is a method of causal analsis. It uses a formal notion of causality and allows the analyst to choose the level of rigour.
WBA is capable of analysing technical and socio-technical systems, independent of the application domain and has been used in a wide variety of system domains. Among others in aviation, marine, industrial processing, railway and in computer security. It is used in industry, in court and in academia.
WBA’s use of a formal notion of causaliy enables the analyst to check analyses for relative-completeness, falsifiability, reproduceability and objectivity.
The result of a WBA is a “Why-Because Graph”. A WBG is a visual representation of cause-effect relationships between factors. It can be easily understood by a non-expert and facilitates understanding of the complexity of cases analysed.
Mathmatically spoken a WBG is a directed, acyclic graph. Each node represents a causal factor. The directed edges represent the cause-effect relations.
To assure the correctness of cause-effect relations the the “Counterfactual Test” is used. The CT is one part of WBA’s formal notion of causality. It goes back to philosophers Davis Hume and David Lewis. The test is done in form of a question. “If the cause had not been, could the effect ever had happened?”. If the answer is yes, then the cause is a “Necesary Causal Factor” (NCF) for the effect. Then only will there be an edge between the factors in the WBG. The CT assures the correctness of the WBG.
To check relative-completeness the “Causal Sufficiency Test” (CST) is used. If an effect has a number of causes (NCFs) then the CST question is asked: “Will an effect always happen if all attributed causes happen?” If the answer is yes then comleteness is assured. Otherwise the analyst learns that there is still something missing.
Both tests operat on small subsets of a WBG. But the test are transive in nature, so if all subgraphs have been checked for the CT and the CST then, automatically, the whole WBG is correct. This is in important issue if an analys, or team of analaysts, have to cope with complexity.