As well as these good corrections to the maths & hence the actual number value of the probability, it's obviously important to notice that even if the odds were 1 in 20 billion, that doesn't mean anything other than chance is necessarily at work. As a scientist friend pointed out, anything that occurs randomly can be expected to occur in clusters sometimes. What would be far more bizarre is if a random event always occurred at regular time intervals, eg if every 2,717th day you were called to Gray close, once only and never in between. It would make it easier for the ambo service to know where to position their cars!