Probability

Will the six finally show up on game night? Is that raffle ticket worth it? And what does a 70 percent chance of rain actually mean? Chance is part of every day, and even though nobody knows the next outcome, you can say surprisingly precisely how likely it is.

A number between 0 and 1

A probability is always a number between 0 and 1. Zero means impossible: a normal die never shows a 7. One means certain: it is guaranteed to show a number from 1 to 6. Everything in between describes how good the chances are. The calculation behind it is simple: probability = favourable outcomes divided by possible outcomes. For rolling a six there is 1 favourable outcome out of 6 possible ones, so 1/6.

Fraction, decimal, percent

The same probability wears many faces. If a bag holds 4 red and 6 blue marbles, the chance of red is 4 out of 10: as a fraction 4/10, as a decimal 0.4, as a percentage 40%. All three say exactly the same thing. Fractions and decimals are best for calculating, while percent is what you usually hear in everyday life. It also lets you read the weather forecast: a 70 percent chance of rain means that on 7 out of 10 days with exactly this weather pattern, measurable rain falls where you are.

More than one favourable outcome

Often several outcomes count as a hit. The probability of rolling an even number? The favourable outcomes are 2, 4 and 6, so 3 out of 6: 3/6 = 0.5. The only requirement is that all outcomes are equally likely, as with a fair die or well shuffled raffle tickets. Then it really is just: count, divide, done.