Fault Trees: Sensitivity of estimated failure probabilities to problem representation

Fault trees represent problem situations by organizing "things that could go wrong" into functional categories. Such trees are essential devices for analyzing and evaluating the fallibility of complex systems. They follow many different formats, sometimes by design, other times inadvertently. The present study examined the effects of varying three aspects of fault tree structure on the evaluation of a fault tree for the event "a car fails to start." The fault trees studied had four to eight branches, including "battery charge insufficient," "fuel system defective," and "all other problems." Major results were as follows: (a) People were quite insensitive to what had been left out of a fault tree, (b) increasing the amount of detail for the tree as a whole or just for some of its branches produced small effects on perceptions, and (c) the perceived importance of a particular branch was increased by presenting it in pieces (i.e., as two separate component branches). Insensitivity to omissions was found with both college student subjects and experienced garage mechanics. Aside from their relevance for the study of problem solving, such results may have important implications for (a) how best to inform the public about technological risks and to involve it in policy decisions and (b) how experts should perform fault tree analyses of the risks from technological systems.