Probability Rules of Thumb

It can be difficult to get an intuitive sense of how likely an event is. With that in mind, I've created the above table to give an easy way to get a feel for the numbers without having to mess around with logarithms or anything. If you have an event with a likelihood of 1 in N, you can look up the probability of it having occurred at least once and multiply N by the listed constant to get the number of attempts you'd need. For example, many feel that an event that occurs with 1 in 100 probability is very rare. To get a 5% chance of it having happened once, you only need .05*100 = 5 trials. For a 50% chance (so just as likely as not), it takes .69*100 = 69 trials, and for a 95% chance of it having happened, it takes 3*100 = 300 trials.

One caveat is that this table is only a reasonable approximation for N above about 10 or so.