What Is Probability?
Probability is a way of measuring how likely something is to happen. We use it every day without even realising it — when we check the weather forecast, decide whether to bring an umbrella, or guess the outcome of a coin toss.
A number between 0 and 1 (or 0% and 100%) that describes how likely an event is to occur. A probability of 0 means the event is impossible, and a probability of 1 means the event is certain.
Think of probability as a scale:
- 0 — Impossible (e.g., rolling a 7 on a standard die)
- 0.25 — Unlikely (e.g., picking a specific suit from a deck of cards)
- 0.5 — Even chance (e.g., flipping heads on a fair coin)
- 0.75 — Likely
- 1 — Certain (e.g., the sun rising tomorrow)
Probability can be written as a fraction, a decimal, or a percentage. For example, the probability of flipping heads is , or , or .
Imagine a jar filled with 10 marbles — 5 red and 5 blue. If you close your eyes and pick one, you have an even chance of getting red or blue. That's a probability of . Now imagine 9 red and 1 blue — suddenly blue is much less likely! Probability helps us put a number on that feeling of "how likely."
Key Vocabulary: Events and Outcomes
Before we dive deeper, let's make sure we understand some important terms.
A process or action that produces results, such as flipping a coin, rolling a die, or drawing a card.
A single possible result of an experiment. For example, when rolling a die, one outcome is getting a 4.
A specific outcome or group of outcomes that we are interested in. For example, "rolling an even number" is an event that includes the outcomes 2, 4, and 6.
The set of all possible outcomes of an experiment. For a standard die, the sample space is {1, 2, 3, 4, 5, 6}.
Experiment: Spinning a spinner with 4 equal sections coloured red, blue, green, and yellow.
- Sample space: {red, blue, green, yellow}
- One outcome: landing on blue
- An event: landing on a primary colour (red, blue, or yellow) — this event contains 3 outcomes
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