Probing the improbable: methodological challenges for risks with low probabilities and high stakes

Abstract

Some risks have extremely high stakes. For example, a worldwide pandemic or asteroid impact could potentially kill more than a billion people. Comfortingly, scientific calculations often put very low probabilities on the occurrence of such catastrophes. In this paper, we argue that there are important new methodological problems which arise when assessing global catastrophic risks and we focus on a problem regarding probability estimation. When an expert provides a calculation of the probability of an outcome, they are really providing the probability of the outcome occurring, given that their argument is watertight. However, their argument may fail for a number of reasons such as a flaw in the underlying theory, a flaw in the modeling of the problem, or a mistake in the calculations. If the probability estimate given by an argument is dwarfed by the chance that the argument itself is flawed, then the estimate is suspect. We develop this idea formally, explaining how it differs from the related distinctions of model and parameter uncertainty. Using the risk estimates from the Large Hadron Collider as a test case, we show how serious the problem can be when it comes to catastrophic risks and how best to address it.

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“…With plausible combinations of these factors, a virus that requires three mutations reaches proportions at which a few respiratory droplet transmissible A/H5N1 viruses are likely to be among the particles excreted. For a virus that requires five mutations, it may only reach such proportions with more extreme combinations of factors, a large number of infected mammals, or if an event occurs that is not encompassed by the model (32). However, it is known that influenza viruses are capable of respiratory droplet transmission in animal models at low infectious doses (33), and that transmission routes other than in respiratory droplets could be important, thus despite the three key current unknowns about transmission (6), even low numbers of excreted respiratory droplet transmissible A/H5N1 virus may be relevant for emergence.…”

Underprotection of Unpredictable Statistical Lives Compared to Predictable Ones

“…When estimating the probability of a very unlikely event, the probability of an inaccurate calculation leading to a substantial underestimate of the risk (due to an error in model or in arithmetic) may exceed the probability of the event estimated by the analyst, making the estimate unreliably low in a way that may not be recognized by the analyst. (7) …”

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