Actuary here. I don&#x27;t have the math at my fingertips, but to confidently estimate the risk of any occurrence you need a significant sample of the event, not just a significant amount of exposure.<p>So for instance, if you are estimating the mortality rate of a 25 year old and the rate for an 85 year old you would be equally confident in each estimate if your data sample contained an equal amount of deaths for each, not an equal amount of life-years.<p>So while we have many days of exposure to terrorism risk, we do not have many occurrences and therefore do not have a confident estimate of the rate of major terror attacks.