Scatterplots (5 of 5)

 

Learning Objectives

  • Use a scatterplot to display the relationship between two quantitative variables. Describe the overall pattern (form, direction, and strength) and striking deviations from the pattern.

Labeling Groups in a Scatterplot

If we graph data from two or more groups in a scatterplot, the relationship between the two quantitative variables can be hidden or unclear. We can use a categorical variable to label groups within the scatterplot, then look for patterns within each group. The relationship may be clearer within each group.

Example

Hot Dogs

A study was conducted by a concerned health group in which 54 major hot dog brands were examined. Using this data, we explore the relationship between sodium content and calories. We begin by making a scatterplot with data from the three types of hot dogs: beef, poultry, and meat (meat is a combination of pork, beef, and poultry).

 

Let’s Summarize

  • The relationship between two quantitative variables is visually displayed using the scatterplot, where each point represents an individual. We always plot the explanatory variable on the horizontal x-axis and the response variable on the vertical y-axis.
  • When we explore a relationship using the scatterplot, we should describe the overall pattern of the relationship and any deviations from that pattern. To describe the overall pattern, consider the direction, form, and strength of the relationship. Assessing the strength just by looking at the scatterplot can be problematic; using a numerical measure to determine strength is discussed later in this course.
  • Adding labels to the scatterplot that indicate different groups or categories within the data might help us gain more insight about the relationship we are exploring.