This lesson is optional because it revisits below-grade-level content learned in grade 7 about chance events and probability. If the pre-unit diagnostic assessment indicates that your students know this material, this lesson may be safely skipped. 

In this lesson, students revisit the idea of chance events and the concept of probability. The probability of a chance event is a number between 0 and 1 that expresses the likelihood of the event occurring.

To determine probability, students should be able to correctly use some statistical terminology. A chance experiment is something that can be repeated with an element of randomness. An outcome is a possible result of a chance experiment. The sample space is the collection of all outcomes for a chance experiment. An event is a set of one or more outcomes for a chance experiment.

Students have an opportunity to practice being precise in their language about probability and rational numbers between 0 and 1 throughout the lesson (MP6).