# Lesson 14

Comparing Mean and Median

### Lesson Narrative

In this lesson, students investigate whether the mean or the median is a more appropriate measure of the center of a distribution in a given context. They learn that when the distribution is symmetrical, the mean and median have similar values. When a distribution is not symmetrical, however, the mean is often greatly influenced by values that are far from the majority of the data points (even if there is only one unusual value). In this case, the median may be a better choice.

At this point, students may not yet fully understand that the choice of measures of center is not entirely black and white, or that the choice should always be interpreted in the context of the problem (MP2) and should hinge on what insights we seek or questions we would like to answer. This is acceptable at this stage. In upcoming lessons, they will have more opportunities to include these considerations into their decisions about measures of center.

### Learning Goals

Teacher Facing

• Choose which measure of center to use to describe a given data set and justify (orally and in writing) the choice.
• Explain (orally) that the median is a better estimate of a typical value than the mean for distributions that are not symmetric or contain values far from the center.
• Generalize how the shape of the distribution affects the mean and median of a data set.

### Student Facing

Let's compare the mean and median of data sets.

### Required Preparation

For The Tallest and Smallest in the World activity, students will need the data on their heights (collected in the first lesson). Consider preparing a class dot plot that shows this data set to facilitate discussions.

For the Mean or Median activity, one copy of the blackline master for each group of 3--4 students cut into cards for sorting and examining.

### Student Facing

• I can determine when the mean or the median is more appropriate to describe the center of data.
• I can explain how the distribution of data affects the mean and the median.

### Glossary Entries

• median

The median is one way to measure the center of a data set. It is the middle number when the data set is listed in order.

For the data set 7, 9, 12, 13, 14, the median is 12.

For the data set 3, 5, 6, 8, 11, 12, there are two numbers in the middle. The median is the average of these two numbers. $$6+8=14$$ and $$14 \div 2 = 7$$.