# Set Operations: Union, Intersection, Complement, and Difference

set is a collection of items.

We denote a set using a capital letter and we define the items within the set using curly brackets. For example, suppose we have some set called “A” with elements 1, 2, 3. We would write this as:

A = {1, 2, 3}

This tutorial explains the most common set operations used in probability and statistics.

### Union Definition: The union of sets A and B is the set of items that are in either A or B.

Notation: A ∪ B

Examples:

• {1, 2, 3} ∪ {4, 5, 6} = {1, 2, 3, 4, 5, 6}
• {1, 2} ∪ {1, 2} = {1, 2}
• {1, 2, 3} ∪ {3, 4} = {1, 2, 3, 4}

### Intersection Definition: The intersection of sets A and B is the set of items that are in both A and B.

Notation: A ∩ B

Examples:

• {1, 2, 3} ∩ {4, 5, 6} = {∅}
• {1, 2} ∩ {1, 2} = {1, 2}
• {1, 2, 3} ∩ {3, 4} = {3}

### Complement Definition: The complement of set A is the set of items that are in the universal set U but are not in A.

Notation: A’  or  Ac

Examples:

• If U = {1, 2, 3, 4, 5, 6} and A = {1, 2}, then Ac = {3, 4, 5, 6}
• If U = {1, 2, 3} and A = {1, 2}, then Ac = {3}

### Difference Definition: The difference of sets A and B is the set of items that are in A but not B.

Notation: A – B

Examples:

• {1, 2, 3} – {2, 3, 4} = {1}
• {1, 2} – {1, 2} = {∅}
• {1, 2, 3} – {4, 5} = {1, 2, 3}

### Symmetric Difference Definition: The symmetric difference of sets A and B is the set of items that are in either A or B, but not in both.

Notation: A Δ B

Examples:

• {1, 2, 3} Δ {2, 3, 4} = {1, 4}
• {1, 2} Δ {1, 2} = {∅}
• {1, 2, 3} Δ {4, 5} = {1, 2, 3, 4, 5}

### Cartesian Product Definition: The cartesian product of sets A and B is the set of ordered pairs from A and B.

Notation: A x B

Examples:

• If A = {H, T} and B = {1, 2, 3}, then A x B = {(H, 1), (H, 2), (H, 3), (T, 1), (T, 2), (T, 3)}
• If A = {T, H} and B = {1, 2, 3}, then A x B = {(T, 1), (T, 2), (T, 3), (H, 1), (H, 2), (H, 3)}