The probability scale

Probability is measured on a scale from 0 to 1:

• 0 means impossible;
• 1 means certain;
• 0.5 is an even chance (50%);
• most events lie somewhere in between.

A probability of 0.75 means a 75% chance: more likely than not. A probability can never be below 0 or above 1.

Picture six cards, some purple. With no purple cards, picking purple is impossible. With all six purple, it is certain. In between, the chance slides from less likely, to even, to more likely.

The book's examples, on the scale:

• rolling more than 6 on a die: impossible;
• rolling a 3: less likely;
• heads on a coin: even chance;
• a card numbered 2 to 10 from a pack of 52 (36 such cards): more likely;
• a red sweet from a bag of only red sweets: certain.

Ranking everyday events needs a reason. Monday always comes after Sunday: certain. Snow in Mumbai in July: impossible, in practice.