# Illustrative Mathematics Grade 8, Unit 6, Lesson 8: Analyzing Bivariate Data

Learning Targets:

• I can analyze a set of data to determine associations between two variables.

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Illustrative Math

#### Lesson 8: Analyzing Bivariate Data

Let’s analyze data like a pro.

Illustrative Math Unit 8.6, Lesson 8 (printable worksheets)

#### Lesson 8 Summary

People often collect data in two variables to investigate possible associations between two numerical variables and use the connections that they find to predict more values of the variables. Data analysis usually follows these steps:

1. Collect data.
2. Organize and represent the data, and look for an association.
3. Identify any outliers and try to explain why these data points are exceptions to the trend that describes the association.
4. Find a function that fits the data well.
Although computational systems can help with data analysis by graphing the data, finding a function that might fit the data, and using that function to make predictions, it is important to understand the process and think about what is happening. A computational system may find a function that does not make sense or use a line when the situation suggests that a different model would be more appropriate.

#### Lesson 8.1 Speed vs. Step Length

A researcher found an association between a dog’s stride length and its speed: the longer a dog’s steps, the faster it goes. The predicted speed in meters per second, s, as a function of step length in meters, l, is
s = 4l - 1.4
What does the rate of change of the function tell you about the association between stride length and speed?

#### Lesson 8.2 Animal Brains

Is there an association between the weight of an animal’s body and the weight of the animal’s brain?
What do you notice in the table of data?
Consider the scatter plot of the data. Are there any outliers?
Experiment with the line to fit the data. Drag the points to move the line. You can close the expressions list by clicking on the double arrow.
Open Applet

1. Without including any outliers, does there appear to be an association between body weight and brain weight? Describe the association in a sentence.
2. Adjust the line by moving the green points, fitting the line to your scatter plot, and estimate its slope. What does this slope mean in the context of brain and body weight?

#### Are you ready for more?

Use one of the suggestions or find another set of data that interested you to look for associations between the variables.

• Number of wins vs number of points per game for your favorite sports team in different seasons
• Amount of money grossed vs critic rating for your favorite movies
• Price of a ticket vs stadium capacity for popular bands on tour
After you have collected the data,
1. Create a scatter plot for the data.
2. Are any of the points very far away from the rest of the data?
3. Would a linear model fit the data in your scatter plot? If so, draw it. If not, explain why a line would be a bad fit.
4. Is there an association between the two variables? Explain your reasoning.

#### Lesson 8.3 Equal Body Dimensions

Earlier in this unit, your class gathered data on height and arm span.

1. Sometimes a person’s arm span is equal to their height. Is this true for anyone in the class?
2. Build a scatter plot of arm span versus height, and describe the association.
3. Is the line y = x a good fit for the data? If so, explain why. If not, find the equation of a better line.
4. Examine the scatter plot. Which person in your class has the largest ratio between their arm span and their height? Explain or show your reasoning.
Open Applet

#### Lesson 8 Practice Problems

1. Different stores across the country sell a book for different prices. The table shows the price of the book in dollars and the number of books sold at that price.
a. Draw a scatter plot of this data. Label the axes.
b. Are there any outliers? Explain your reasoning.
c. If there is a relationship between the variables, explain what it is.
d. Remove any outliers, and draw a line that you think is a good fit for the data.
2. Here is a scatter plot:
Select all the following that describe the association in the scatter plot:
A. Linear association
B. Non-linear association
C. Positive association
D. Negative association
E. No association
3. Using the data in the scatter plot, what can you tell about the slope of a good model?
A. The slope is positive.
B. The slope is zero.
C. The slope is negative.
D. There is no association.

The Open Up Resources math curriculum is free to download from the Open Up Resources website and is also available from Illustrative Mathematics.

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