Describing Scatterplots

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SWBAT describe the direction, form and strength of a relationship between two quantitative variables.

Big Idea

The appearance of a scatter plot can tell us a lot about the relationship between the variables on the axes.

Opening Activity

15 minutes

As a warm-up, I ask my students to complete Review of Bivariate Data Displays, a worksheet that contains a summary of the previous day's discussion.  They work independently to fill in the blanks in this worksheet, testing their recall of the previous day's lesson.



Foldable Notes

30 minutes

We have spent some time discussing the various types of two variable data sets that we might come across (2 categorical, 2 quantitative, or one of each) and how to display each type.  At this point, we focus in on data sets that can be displayed with a scatter plot. 

I want my students to remember that the most important features of a scatter plot are Direction, Outliers, Form and Strength. To help them remember this, I use the mnemonic DOFS:

  • D = the direction of the association between the two variables (positive or negative)
  • O = whether or not there are outliers in the data set
  • F = the form of the association (linear or curved)
  • S = a measure of strength, or how close the dots are to the line or curve

In this lesson, we make a Foldable Resource for their notes that details best practices for describing these four characteristics of a bivariate data set.  

Practice Describing Relationships

20 minutes

For the next activity, I ask students to work in the pairs established in the previous day's lesson to  practice describing scatterplots.  Using their foldable notes and the Data Display Cards  they sorted yesterday, my students will practice verbally describing the scatter plots to their partner.  As they do this, I circulate around the room listening to the discussions and making notes about their progress [MP3].

 I do not yet ask students to write down formal descriptions of these scatter plots because we have not yet discussed residuals and correlation coefficients.  In order to do a thorough job of describing the scatter plots they will need to use these concepts, so I do not want them to get used to writing descriptions that omit them.

Closing Discussion and Assignment

20 minutes

When students have had 20 minutes to practice describing the data sets, I ask for volunteers to describe the scatter plots as I display them with a projector.  I ask the class to critique their description and provide feedback.

The homework for this evening will be a "throwback" assignment, WS Writing Equation of a Line. In this assignment, students are asked to write the equation of a line given various starting information.  This should be a pretty relaxing assignment because students spent a lot of time on linear functions in Algebra 1.  In the next lesson, we will use regression equations to make predictions so I want students to be familiar with how the equation of a line is determined.