# What is a simple event in probability? What is a complementary event?

##### 1 Answer

See explanation.

#### Explanation:

A **elementary event** is (exactly speaking) a primary oobject which is not defined (as for example a point or a number).

More generally it is a "basic result" of a probability experiment. For example if a die is thrown the possible results (i.e. elementary events) are

A set of all elementary events is called **a sample space** and denoted by

An **event** is any subset of

A **complementary event** of

so the complementary event consists of all elementary events which are **not** in the event

**Example**

I wrote earlier about an experiment of a die throw. Its sample space is (as I wrote) a set of numbers

Let *a prime number is thrown*.

Then

A complementary event would be *a number which is not prime is thrown*, so