There are many instances where we want to explore the relationship between two variables. If one of the variables is categorical, and the other is quantitative, we could construct bar graphs or pie charts to help identify the relationship between them.
When both of the variables we wish to work with are quantitative, the data is called bivariate. Scatterplots are useful for identifying relationships in bivariate data.
In considering the relationship between two quantitative variables, we can sometimes identify one of the variables as the explanatory variable, or independent variable, and the other as the response variable, or dependent variable.
The response variable is so named because it depends on or is explained by the explanatory variable. Sometimes it might not be obvious which is the explanatory variable and which is the response variable.
In a scatterplot of bivariate data, we use the horizontal or "x-axis" for the explanatory variable and the vertical, or "y-axis" for the response variable.
Look at Example 2.2, on pages 33-34 in the textbook, for some practice matching descriptions to scatterplots.