Which of the following is the primary use of a scatter plot?

• A. To show distribution of a single variable
• B. To illustrate time series
• C. To display frequency counts of categories
• D. To examine the relationship between two quantitative variables

Answer: (D)

Scatter plots are used to examine the relationship between two quantitative variables. Each point represents a pair of values, with one variable on the x-axis and the other on the y-axis. If the points form an upward pattern, the two variables tend to increase together; a downward pattern means one tends to decrease as the other increases. The tightness of the pattern indicates how strong the relationship is, and any outlier points stand apart from the rest.

This setup is different from showing how a single variable is spread out (that’s a histogram or box plot), from illustrating how data change over time (that’s a time-series or line graph), or from displaying how often categories occur (that’s a bar chart). So a scatter plot’s main purpose is to reveal how two numerical variables relate to each other.