Most variables in a data set are either numerical or categorical.

The distinction between discrete and continuous numerical variables is important, but that between ordinal and nominal variables is less so.

Warning!

Categorical variables are sometimes coded as numbers when the data are recorded (e.g. gender may be coded as 0 for males and 1 for females). This coding does not make them numerical.

In a similar way, an ID number may be used to identify individuals. It is still a label variable, not numerical.

When you see a column of numbers in your data matrix, do not assume that it is a numerical variable.

Characteristics of patients

European countries

The following data were obtained from the World Bank (http://www.worldbank.org/data) and the World Health Organisation (http://www3.who.int/whosis/menu.cfm).

Note that the some values are unknown (shaded in grey on the map).