Simple Probability

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Probability is the likelihood that something will happen. When the weather forecast says there is a 60% chance of rain, it is expressing a probability. It is more likely that it will rain, but far from certain: if this forecast is accurate, 4 out of 10 times it will not rain.


A simple probability can be expressed as the number of ways a certain outcome can occur divided by the number of possible outcomes. For example, consider the probability of rolling a 3 with a six-sided die. There is only one way to roll a three, and there are six possible outcomes of rolling the die. Therefore, the probability is 1 out of 6, which can also be expressed as 1/6 or roughly 17%. Probabilities range from 0 to 1, with 0 being something that will never happen (rolling a 7 with a six-sided die) and 1 being something that will always happen (rolling a 1, 2, 3, 4, 5, or 6).


In this opening look at probabilities, we will stick to simple scenarios. There are a couple of terms you should know, however. In probability, an experiment is a repeatable process that has a limited number of possible results. These possible results are called outcomes, and it should be possible to identify and count all of them. Finally, an event is one or more outcomes of the experiment. Note that events can include more than one outcome: for instance, an event might be rolling an odd number, which includes the outcomes of rolling a 1, 3, or 5.