Probability and probability distributions

Probability measures how likely an event is, on a scale from 0 (impossible) to 1 (certain). The classical view counts equally likely outcomes; the empirical view uses observed frequencies. Worked example: drawing one card from a standard pack, the probability of an ace is 4 ÷ 52 = 1/13, and the probability of a king or a queen is 8 ÷ 52 = 2/13, because the two events are mutually exclusive and their probabilities simply add.