## The search for MH370: uncertainty

There is an interesting podcast about the search for MH370 by a former colleague. I think it illustrates in a relatively accessible form some aspects of uncertainty.

According to the familiar theory, if one has an initial probability distribution over the globe for the location of MH370’s flight recorder, say, then one can update it using Bayes’ rule to get a refined distribution. Conventionally, one should search where there is a higher probability density (all else being equal). But in this case it is fairly obvious that there is no principled way of deriving an initial distribution, and even Bayes’ rule is problematic. Conventionally, one should do the best one can, and search accordingly.

The podcaster (Simon) gives examples of some hypotheses (such as the pilot being well, well-motivated and unhindered throughout) for which the probabilistic approach is more reasonable. One can then split one’s effort over such credible hypotheses, not ruled out by evidence.

A conventional probabilist would note that any ‘rational’ search would be equivalent to some initial probability distribution over hypotheses, and hence some overall distribution. This may be so, but it is clear from Simon’s account that this would hardly be helpful.

I have been involved in similar situations, and have found it easier to explain the issues to non-mathematicians when there is some severe resource constraint, such as time. For example, we are looking for a person. The conventional approach is to maximise our estimated probability of finding them based on our estimated probabilities of them having acted in various ways (e.g., run for it, hunkered down). An alternative is to consider the ways they may ‘reasonably’ be thought to have acted and then to seek to maximize the worst case probability of finding them. Then again, we may have a ranking of ways that they may have acted, and seek to maximize the number of ways for which the probability of our success exceeds some acceptable amount (e.g. 90%). The key point here is that there are many reasonable objectives one might have, for only one of which the conventional assumptions are valid. The relevant mathematics does still apply, though!

Dave Marsay