ABSTRACT
Probability is merely a way of expressing the likelihood of an event. The idea of probability is used extensively in Environment Health and Safety. An expected value (EV) is a weighted average value, based on probabilities. Two common methods using expected value to present various decision options and their possible outcomes are in the forms of pay-off table and the decision tree. A pay-off table is simply a table that lists the various options, possible outcomes, and results. Probability distribution describes how the probabilities are distributed over the values of random variables. The Binomial, Poisson, and Hyper-geometric probability distributions are three discrete distributions that describe many common physical phenomena. The hyper-geometric probability distribution is closely related to the binomial probability distribution. Three continuous probability distributions will be discussed under this heading: normal, uniform, and exponential. The continuous exponential distribution is often used to describe the probabilities of occurrences of an event in terms of space and time.
