## Simple Probability

**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.