 Then, the probability that the event does happen, at least once, in 1000 trials is or 63.23%.This means that this "unlikely event" has a probability of 63.23% of happening if 1000 independent trials are conducted, or over 99.9% for 10,000 trials.The probability that it happens at least once in 10,000 trials is In other words, a highly unlikely event, given enough trials with some fixed number of draws per trial, is even more likely to occur.