Lesson 14: Variables, Variables, Variables

Objective

You will learn how to read and interpret multiple variable plots: bivariate scatterplots, multiple variable scatterplots, stacked bar plots, and side-by-side bar plots. You will summarize your learning about multiple variable plots using a four-fold graphic organizer.

Vocabulary

scatterplot, grouping, side-by-side bar plot

Essential Concepts

Lesson 14 Essential Concepts

To examine whether two (or more) variables are related, we can plot their distributions on the same graph.

Lesson

In this lesson you will learn how to make visual displays using more than one variable. Begin by pondering the following questions:
1. What do you think the relationship is between people's heights and weights?
2. Are taller people heavier? Always? Or is this just a tendency?
Let's look at the following plot, which has real data relevant to these questions.

Click on the document name to download a fillable copy of the Scatterplot of Heights vs. Weights handout (LMR_1.13).
In your IDS Journal, answer the following questions about the plot from the handout:
1. What kind of plot is this and how will you remember its features?
2. How many variables are displayed in this plot? Name the variable(s) and identify the type(s) of variable(s).
3. What do the axes show?
4. Do taller people weigh more?
Check your understanding of this first plot. Here are some possible responses:
1. Scatterplot.
2. Two variables. Weight in kilograms and height in meters. Numerical variables.
3. The x-axis shows the height of teens in meters, and the y-axis shows the weight of teens in kilograms.
4. Not necessarily, but there is a tendency for this to be true.
  
  Confirm, correct, or add to your own responses.
Now that you have identified some components of this plot, what additional questions might you have about it? What additional information would be helpful?
Now suppose we could see which of these dots represented girls and which represented boys.

Where do you think most of the girls' dots would be relative to the boys?
Look at the following plot that addresses this question.

Click on the document name to download a fillable copy of the Scatterplot of Heights vs. Weights by Gender handout (LMR_1.14).
In your IDS Journal, answer the following questions about the plot from the handout:
1. What kind of plot is this and how will you remember its features?
2. How many variables are displayed in this plot? Name the variable(s) and identify the type(s) of variable(s).
3. What do the axes show?
4. What questions can we ask that this graph might answer?
Check your understanding of this plot. Here are some possible responses:
1. Scatterplot.
2. Three variables. Weight in kilograms and height in meters are numerical variables. Gender is categorical.
3. The x-axis shows the height of teens in meters, and the y-axis shows the weight of teens in kilograms.
4. Some questions might be "Who is taller, boys or girls?", "Who weighs more?", or "Is the association between the height and weight the same for boys as it is for girls?".
  
  Confirm, correct, or add to your own responses.
  
  Note: When the data are split into categories, it is called grouping.
Now that you have explored some components of this plot, what questions might you have about it? What additional information would be helpful?
Let's look at another type of plot.

Click on the document name to download a fillable copy of the Side-by-side Barchart handout (LMR_1.15).
In your IDS Journal, answer the following questions about the plot from the handout:
1. What kind of plot is this and how will you remember its features?
2. How many variables are displayed in this plot? Name the variable(s).
3. What are the x-axis and y-axis telling us?
4. What statistical questions can you answer with this graph?
Check your understanding of this plot. Here are some possible responses:
1. Side-by-side bar plot.
  
  Note: The terms side-by-side bar plot, side-by-side bar chart, and side-by-side bar graph can be used interchangeably.
2. Two variables. Whether or not someone is Hispanic, and how often they wear sunscreen.
3. The x-axis shows how often a student wears sunscreen, and the y-axis shows the percentage of the total that fall into that category (broken into two bars, one for Hispanic and one for non-Hispanic).
4. Some questions might be "Do Hispanics and non-Hispanics have different approaches to sunscreen?", "What percent of Hispanics always/never wear sunscreen?", or "How does that compare to non-Hispanics?"
  
  Confirm, correct, or add to your own responses.
After exploring this plot, what additional questions might you have about it? What additional information would be helpful?
Now look at the following plot.

Click on the document name to download a fillable copy of the Faceted Histogram of Height by Gender handout (LMR_1.16).
In your IDS Journal, answer the following questions about the plot from the handout:
1. What kind of plot is this and how will you remember its features?
2. How many variables are displayed in this plot? Name the variable(s).
3. What are the x-axis and y-axis telling us?
4. What statistical questions can you answer with this graph?
Check your understanding of this plot. Here are some possible responses:
1. Split histogram (you learned about a split histogram in Lab 1B).
Note: The terms faceted histogram and split histogram will be used interchangeably.
1. Two variables. Height and gender.
2. The x-axis shows height in feet, and the y-axis shows the total that fall into a certain range of heights (broken into two histograms, one for males and one for females).
3. Some questions might be "Do males and females differ in height?", "What is the typical female height?", or "What is the typical male height?".
  
  Confirm, correct, or add to your own responses.
Using the notes and sketches in your IDS Journal, summarize your learning of how to read and interpret basic multiple variable plots by completing the Summarizing Multiple Variable Plots fourfold graphic organizer.

Note: If you are unable to print the handout or access it electronically, you can create a table in your IDS Journal that follows the same format.

Click on the document name to download a fillable copy of the Summarizing Multiple Variable Plots fourfold graphic organizer (LMR_1.17).

Reflection

What are the essential learnings you are taking away from this lesson?

Next 2 Days

LAB 1D: Zooming through Data

LAB 1E: What’s the Relationship?

Complete Lab 1D and 1E prior to the Practicum.