What is Probability?
Probability is the branch of mathematics that deals with how likely something is to happen. We encounter probability every day — when we check the weather forecast, play a board game, or wonder whether our toast will land butter-side down.
A measure of how likely an event is to occur, expressed as a number between 0 and 1 (or equivalently, between 0% and 100%).
- A probability of 0 means the event is impossible (it will never happen).
- A probability of 1 means the event is certain (it will definitely happen).
- Most events fall somewhere in between.
Think of probability like a volume dial on a speaker. At 0 there's no sound at all (impossible), at 1 the volume is at maximum (certain), and most of the time you're somewhere in the middle — just like most events have a probability between 0 and 1.
Probability can be written as a fraction, a decimal, or a percentage:
- = =
All three forms mean the same thing: the event has an equal chance of happening or not happening.
Key Vocabulary: Experiments, Outcomes, and Events
Before we calculate probabilities, we need to understand some essential terms.
Any process that produces a well-defined result, such as rolling a die, flipping a coin, or drawing a card from a deck.
A single possible result of an experiment. For example, getting a 4 when rolling a die is one outcome.
A set of one or more outcomes that we are interested in. For example, "rolling an even number" on a die is the event {2, 4, 6}.
Experiment: Spinning a spinner with sections labelled Red, Blue, Green, Yellow.
Possible outcomes: Red, Blue, Green, Yellow
Event — "landing on a warm colour": {Red, Yellow}
Always list your outcomes carefully before jumping into calculations. Clearly identifying every possible outcome prevents mistakes later!
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