Most variables in a data set are either numerical or categorical.
Numerical variables
These have values that are numbers and can be further classified into:
Statistical methods that can be used for continuous variables are not always appropriate for discrete variables.
Categorical variables
The values of a categorical variable are selected from a small group of categories. A further classification is:
Most statistical methods for categorical data can be applied to both ordinal and nominal variables.
Labels
In some data sets, each individual has a unique 'name' that can be used to identify it. We call this a label variable.
Warning!
When you see a column of numbers in your data matrix, do not assume that it is a numerical variable.
Numbers are sometimes used as codes for categorical or label variables.