## SECTION 4.2 Summary

- A
**probability model** for a random phenomenon consists of a sample space and an assignment of probabilities .
- The
**sample space** is the set of all possible outcomes of the random phenomenon. Sets of outcomes are called **events.** assigns a number to an event as its probability.
- The
**complement** of an event consists of exactly the outcomes that are not in .
- Events and are
**disjoint** if they have no outcomes in common.
- Events and are
**independent** if knowing that one event occurs does not change the probability we would assign to the other event.
- Any assignment of probability must obey the rules that state the basic properties of probability:
**Rule 1.** for any event .
**Rule 2.** .
**Rule 3. Addition rule:** If events and are **disjoint,** then .
**Rule 4. Complement rule:** For any event ,
.
**Rule 5. Multiplication rule:** If events and are **independent,** then .