The first step, before any calculations or plotting of data, is to decide what type of data one is dealing with. There are a number of typologies, but one that has proven useful is given in Table 1.1 .
Quantitative variables can be continuous or discrete.
Continuous variables, such as height, can in theory take any value within a given range. For nominal variables with more than two categories the order does not matter. In this case the order does matter and it is usually important to account for it. In general it is easier to summarise categorical variables, and so quantitative variables are often converted to categorical ones for descriptive purposes. It may be easier to think of the proportion of the population who are hypertensive than the distribution of blood pressure.