Chance and randomness

Probability is a kind of measurement. A ruler measures length; probability measures how likely something is: how sure we can be that it will happen. It turns 'maybe' into a number.

"Will it rain today? Will our team win tomorrow? Will my name come out in the lucky draw?" These are random events: we know what could happen, but not which one will.

Two ways to judge chances:

• 'the sun is shining, so it probably won't rain' is a subjective judgement: a feeling;
• this chapter is about objective ways, from evidence and from reasoning.

A random experiment can be repeated; each time the result may be different and cannot be known in advance:

• tossing a coin: heads or tails;
• rolling a die: 1 to 6;
• drawing a name from folded slips.

Randomness can be useful. A coin toss decides who bats first in cricket because neither captain can control it or predict it, so both have the same chance.

Rain is random because it depends on many sensitive things: temperature, humidity, wind, pressure. We cannot know it for certain, but we can estimate its probability from data.