In the absence of more compelling evidence, an appropriate starting point for the exposure estimation may be the fact that a component or collection of components has not failed after x years in service. This involves the notion of having ‘withstood the test of time’. A component having survived a threat, especially for many years, is evidence of the exposure level. This is best illustrated by example. If 10 miles of pipe, across an area with landslide potential, has been in place for 30 years without experiencing any landslide effects, then a failure tomorrow perhaps suggests an event rate of 1/(10 miles x 30 years) = 1/300 mile years.
This simple estimate will not address the conservatism level. The estimator will still need to determine if this value represents more of a P50 estimate or perhaps a more conservative P90+ value.
In some cases, the evidence is actually of the mitigated exposure level. That is, the component has survived the threat, but perhaps at least partially due to the presence of effective mitigation. This makes the separation of exposure more challenging.
Despite the lack of complete clarity, this ‘test of time’ rationale can be a legitimate part of an exposure estimate.