4 Numerical Data
The distinctive characteristic of numerical data is the fact that we can calculate the difference between measurements. While in categorical data one can only differentiate points, knowing whether they are the same or not, and in ordered data we can also compare them, knowing which one is bigger or smaller, with numerical data with can also know how big is the difference between these points. In short, for points x₁ and x₂ one should be able to calculate x₂ − x₁.
But just being able to do a subtraction doesn’t mean your data is numeric. For example, it doesn’t make sense to make the difference between diabetes type 1 and type 2 to obtain 1. So how do you know if your data is numeric? Basically, the difference between measurements should make physical sense. A good indication of that is the difference having a measurement unit.