Probability is the likelihood of an event happening. If the possible events can be counted, then the probability of a specific type of event is calculated as the number of all the events of that type divided by the total number of possible events. For a fair coin, the probability that a flipped coin will land heads is 1/2 = .5 because one of the two possible outcomes matches the condition. For a four-alternative multiple choice question, the probability of guessing the correct answer equals 1/4 = .25 because one of the four possible outcomes matches the condition. Probability is useful in statistics for helping us detect unusual or rare events. Formulas for the binomial distribution provide the probabilities whenever there are multiple trials with only two possible outcome on each trial.