Consider the experiment of throwing a die. Let E denotes the event “ a number less than 4 appears”. If actually ‘1’ had appeared on the die then we say that event E has occurred. As a matter of fact if outcomes are 2 or 3, we say that event E has occurred. Thus, the event E of a sample space S is said to have occurred if the outcome ω of the experiment is such that ω ∈ E.