Quantitative and Qualitative Data

All data is either quantitative – measured with some kind of measuring implement, such as a ruler, jug, weighing scales, stopwatch, thermometer, and so on – or is qualitative: an observed feature of interest that is placed into categories, as in health (healthy, sick), and opinion (agree, neutral, disagree).

Quantitative and qualitative data can be sub-divided into four further classes of data – Ratio, Interval, Ordinal, and Nominal – as shown in Figure 2.1.

Figure 2.1: There are four distinct types of data

The differences between them can be established by asking just three questions:

Question 1: Are adjacent data points or categories ordered?

All measured data is ordered, but not all categories are. If your categories are named [Small; Medium; Large], then there is an order to them. If you have named your categories [1; 2; 3], then there may be an order, but it all depends on what 1, 2, and 3 signify. Just because...