In high school, we learned about set representations of events. Below we will introduce the set representation of events based on the sample space.

　　By definition, sample space is the set of all possible results ( sample points ) of a random experiment, and each sample point is an element of the set. An event is composed of those possible outcomes that have the characteristics required by the event, so an event is a set corresponding to the sample points with corresponding characteristics in , which is a subset of . Therefore, any event can be represented by a certain subset of .
　　When we say that an event occurs, it means that a certain sample point belonging to the event appears in the random experiment.
　　 For example: In the experiment of throwing dice , the sample space is

.

Therefore,
　　event : " the number of points is " can be expressed as ;
　　event : " the number of points is less than " can be expressed as ;
　　event : "the number of points is an even number less than " can be expressed as .
　　We call an event that contains only one sample point a basic event; an event that contains two or more sample points is a composite event. Obviously, sample space as an event is a necessary event, and empty set as an event is an impossible event.