What must be true of all the probabilities listed in a probability model?

1. A probability model lists every possible outcome together with its probability. 2. Since one of those outcomes is certain to occur, their probabilities must account for the whole of certainty. 3. So the listed probabilities add up to 1, or equivalently 100%. 4. They need not be equal, as a 1% chance of winning a raffle sits beside a 99% chance of losing it. 5. They may also be written as fractions, decimals or percents, so no single format is required. _Source: OpenStax Algebra and Trigonometry (CC BY 4.0), Ch 13 "Sequences, Probability and Counting Theory", section 13.7 Probability_