• Describe events as subsets of a sample space (the set of outcomes) using characteristics of the outcomes or as unions, intersections, or complements of other events (“or”, “and,” “not”) (S-CP.A.1). • Develop the Addition Rule to compute probabilities of compound events in a uniform probability model (S-CP.B.7). • Apply the Addition Rule in a uniform probability model and interpret the answer in terms of the model (S-CP.B.7).

In grade 7, students described sample spaces by creating lists, tables, and diagrams. They determined P(A or B) by counting occurrences of simple events in A ∪ B. In this section, students learn to calculate P(A or B) in terms of P(A), P(B), and P(A and B). They begin by considering compound events as subsets of sample spaces, noting that in the sample space: the event “not A” is the complement of A; that the event “A and B” is the intersection of sets A and B; and that the event “A or B” is the union of A and B. The section culminates in a task where the Addition Rule is used to compute a probability.

1 Describing Events

WHAT: Students list events in a sample space and describe unions, intersections, or complements of given compound events.

WHY: Being able to view events as unions, intersections, or complements of given compound events is a preliminary for calculation of the probability of one event in terms of probabilities of other events.