Correlation between numeric variables

The first thing we generally do to understand whether there is an association between two numeric variables is to represent them on the two Cartesian axes in order to obtain a scatterplot:

Figure 11.1 – A simple scatterplot

Using a scatterplot, it is possible to identify three important characteristics of a possible association:

• Direction: It can be positive (increasing), negative (decreasing), or not defined (no association found or increasing and decreasing). If the increment of a variable corresponds to the increment of the other, the direction is positive; otherwise, it is negative:

Figure 11.2 – Direction types of the association

• Form: It describes the general form that association takes in its simplest sense. Obviously, there are many possible forms, but there are some that are more common, such as linear and curvilinear (nonlinear) forms:

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