ABSTRACT

We explain the concept and measurement of probability, distinguishing between judgemental, experimental (relative frequency) and theoretical approaches to determining probabilities of events. We contrast simple probabilities with compound probabilities prior to explaining how to find ‘and’ probabilities based on the intersection of outcomes and the use of the multiplication rule, and ‘or’ probabilities based on the union of outcomes and the use of the addition rule. We examine the implications of outcomes being mutually exclusive and/or collectively exhaustive on determining probabilities, and how to ascertain dependency between outcomes using probabilities. We outline and demonstrate the use of Bayes’s rule, contrasting a posterior or ‘after the event’ probability with a priori or ‘before the event’ probability. We conclude by showing how to use probability trees to analyse a series of events with different outcomes.