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.
As a probability is closer to 0, the less likely an event is to happen. As a probability is closer to 1, the more likely an event is to happen.

Expressing Probabilities

A probability can be expressed as:

Lesson Slides

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A Note on Notation

The probability of an event can be written as:
P(Event)
A letter or symbol can be used to represent an event. For example, let H be the event that a coin lands on Heads when it has been tossed. We can denote the probability of getting heads as:
P(H)