probability

Everyday

1. (maths) How likely something is to happen, given as a number from 0 (impossible) to 1 (certain), or as a fraction or percentage. tree →
2. The chance that something will happen: «There's a high probability of rain.» tree →
3. (formal) «In all probability» means «very likely». tree →

In the lessons

In maths lessons probability uses words like impossible, unlikely, even chance, likely, certain. A fair coin has probability 1/2 of heads; a dice has probability 1/6 of rolling a 3. Probability = number of ways it can happen ÷ total number of equally likely outcomes. Weather forecasts give the probability of rain. Phrases: «the probability of», «high/low probability», «in all probability», «probability scale».

Examples

• The probability of getting heads is one half. tree →
• What is the probability of rolling a six on a dice? tree →
• There's a high probability of snow tomorrow. tree →
• In all probability, the bus will be late. tree →
• Draw an arrow on the probability scale to show «unlikely». tree →
• If a bag has 3 red and 1 blue marble, the probability of red is 3/4. tree →
• The probability of an impossible event is 0. tree →
• It is only a probability, but we cannot in any case reach more than probability in this matter, just at present. tree →

Note

Countable and uncountable. Adjective: probable; adverb: probably. In maths, probability is never less than 0 or more than 1; all the probabilities of the possible outcomes add up to 1. Trap: «probably» (likely) is not «certainly».