Probability is the likelihood that an event will happen. The probability of an event is always between zero and one, or between 0% and 100%. An event with a probability of zero is impossible, and an event with a probability of one is certain. Events with a probability closer to zero are unlikely, while those with a probability closer to one are likely. Events with the same probability are equally likely.
The probability of an event can be written as , which is read as "the probability of the event." In an experiment that involves flipping a coin, the notation or may be used to denote the probability that the outcome is heads.
The probabilities of all the outcomes in the sample space of an event have a sum of 1 because it is certain that one of the outcomes will happen. The complement of an event is all outcomes in the sample space that are not the event. For example, if an event is rolling an even number on a die, then the complement of that event is not rolling an even number, which is the same as rolling an odd number.The notation means that the probability of is the value . The sum of the probabilities of an event and its complement is 1.
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