## The Lesson

A probability is a measure of how likely (how*probable*) an event is to happen. A probability is expressed as a number between 0 (impossible) and 1 (certain).

## Dictionary Definition

The Oxford English Dictionary defines a probability as "the extent to which a particular event is likely to occur... expressed by a number between 0 and 1.## The Probability Scale

Probabilities express how likely an event is to happen on a scale of 0 to 1.- An event with a probability of 0 is impossible: it can not happen.
- An event with possibility of 1 is certain: it will happen.

## Expressing Probabilities

A probability can be expressed as:- a number between 0 and 1.
- a fraction.
- a percentage between 0% and 100%.

## A Note on Notation

The probability of an event can be written as:*P(Event)*

**H**be the event that a coin lands on

**H**eads when it has been tossed. We can denote the probability of getting heads as:

*P(H)*