The activities in this optional lesson all address the concept of measurement error. The activities build on each other and increase in difficulty, but each activity can stand on its own, and not all activities must be completed for students to benefit from the work. As students move through the activities, they build from error in a single measurement to adding measurements to two- and three-dimensional measurements. Students look for patterns and use repeated reasoning as they complete similar mathematical work in increasingly challenging problems (MP8). As they make decisions about percent error, students attend to precision by evaluating the degree of certainty to use when reporting their answer (MP6). This lesson relies on skills developed in Unit 4 and Unit 6.