Correlation between numeric variables
The first thing we generally do to understand whether there is an association between two numeric variables is to plot them on the two Cartesian axes to obtain a scatterplot:
Figure 15.1: A simple scatterplot
Using a scatterplot, it is possible to identify three important characteristics of a possible association:
- Direction: This can be positive (increasing), negative (decreasing), or not defined (no association found – or both increasing and decreasing at the same time). If the increment of one variable is in accordance with the increment of the other, the direction is positive; if the increment of one variable is in accordance with the decrement of the other, it is negative; otherwise, it is not defined:
Figure 15.2: Direction types of the association
