A.1.4—Simple probability

Probability lies between 0 and 1 and describes how often an outcome is expected under stated conditions. Define the event and the sample space before calculating.

For equally likely outcomes, divide favourable outcomes by total outcomes; for repeated independent events, multiply their probabilities only when independence is justified.

If 3 of 12 sampled cells meet a criterion, the observed proportion is 0.25; it is evidence from that sample, not a guarantee for every future cell.

A low probability is not proof that an event is impossible, and “at least one” is not the same as “exactly one”.