Probability Rules

We start with four basic rules of probability. They are simple, but you must know them. Rule 1: All probabilities are numbers between 0 and 1.

Rule 2: All probabilities in a sample space must add to 1.

Rule 3: The complement of an event is everything in the sample space except that event. If A is an event, then its complement is written A c. P(A c )=1-P(A)

Rule 4: Two events are disjoint if they do not intersect. (This is also called mutually exclusive.) If A and B are disjoint, P(A or B)= P(A) + P(B). This can also be written as P(A  B)= P(A) + P(B). This is called the addition rule for disjoint events.