Scatterplots (1 of 5)

Example

Highway Signs

A research firm conducts a study to explore the relationship between a driver’s age and the driver’s ability to read highway signs. The subjects are a random sample of 30 drivers between the ages of 18 and 82. (Source: Jessica M. Utts and Robert F. Heckard, Mind on Statistics [Brooks/Cole, 2002]. Original source: Data collected by The Last Resource, Inc., Bellfonte, PA.)

Because the purpose of this study is to explore the effect of age on the driver’s ability to read highway signs,

• the explanatory variable is age, and
• the response variable is the maximum distance at which the driver can read a highway sign, or maximum reading distance.

Both variables are quantitative.

Here is what the raw data look like:

In this data set, the individuals are the 30 drivers. For each driver, we have two values: age and maximum reading distance.

To explore the relationship between age and distance, we create a graph called a scatterplot. To create a scatterplot, we use an ordered pair (x, y) to represent the data for each driver. The x-coordinate is the explanatory variable: driver’s age. The y-coordinate is the response variable: maximum reading distance.

For this example, the ordered pair (18, 510) represents an 18-year-old driver who can read a highway sign at a maximum distance of 510 feet. We plot a point for each ordered pair. In the scatterplot, each driver appears as a single point.

Generally, each point in a scatterplot represents one individual. The x-coordinate is the value of the explanatory variable for that individual. The y-coordinate is the value of the response variable for that individual.

Here is the completed scatterplot: