> I'm confused by the 'likely to create an event in the next 30 years' part - either it's unlikely or it's likely. Let's suppose the event is an accident occur

> I'm confused by the 'likely to create an event in the next 30 years' part - either it's unlikely or it's likely. Let's suppose the event is an accident occurring during some industrial process. This process is going to happen very frequently over the next 30 years. While the probability of an accident during any particular instance of the process may be low, the probability of the accident happening at least once during that period may be quite high. Some math: p -- accident probability n -- # of times the process happens in the period under consideration Probability of accident happening at least once: 1 - (1 - p)^n For example, suppose the process fails 0.01% of the time (1/10000) and happens twice a year for 30 years. Then an accident happens 1 - (0.9999)^60 ~= 0.6% of the time. If the failure rate is 1%, the probability of at least one failure jumps to 1 - (0.99)^60 ~= 45%.