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Relationships Between Two Quantitative Variables (Lesson 3.2)

Chapter 3 - Day 2

Learning Targets
  • Make a scatterplot to display the relationship between two quantitative variables.
  • Describe the direction, form, and strength of a relationship displayed in a scatterplot, and identify outliers.

Activity: How Many Rubber Bands Does Barbie Need? 

Experience First

Each group of students will choose one Barbie (or action figure) for their group. They will collect data about the distance the Barbie travels for a bungee jump based on the number of rubber bands attached. This Barbie Bungee context will be used for this lesson and for lessons 3.4 and 3.5. At the end of the Chapter in the Barbie Bungee Finale, the students will need to predict the number of rubber bands for the final bungee jump. 


Here are a few tips for making data collection go quickly:

  • Tape meter sticks to the wall at several locations throughout the room.

  • Inform students to use the slow-mo video feature on their phones to help them find the lowest point that Barbie reaches during the jump. 

  • Remind students that distance travelled is measured from the jumping level to the lowest point that Barbie’s head reaches during the jump.

Formalize Later

Don’t expect students to know exactly what to write when describing the relationship in #4. We will help them formalize their thinking during the debrief of the activity. By the end of the lesson, students will know that they should describe the Direction, Outliers, Form, and Strength (some teachers use DOFS as an acronym to remember these). 


Earlier in Chapter 1, we had students describe a distribution using VSCO (Variability, Shape, Center, Outliers). Notice that we ask students to describe a distribution when there is one quantitative variable and to describe a relationship when there are two quantitative variables. Here is a summary:

  • One-variable. Describe the distribution. VSCO (Variability, Shape, Center, Outliers).

  • Two-variable. Describe the relationship. DOFS (Directions, Outliers, Form, Strength).


As for describing the strength of a linear relationship between two quantitative variables, students can use words like “weak”, “moderate”, or “strong” for today’s lesson. They might wonder how they would know which word to choose. We will explore this question in the next lesson when we introduce the correlation r as a numerical measure of the strength of a linear relationship. 

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