interval data

Data which show both the order of magnitude (10², 10³, and so on) and the degree of magnitude. Similarly, an interval scale shows the exact quantities of the variables, or the frequency of their occurrences, and interval level measurements are measurements made such that it is possible to assess the size of the differences between them. For example, if there are three towns with populations of 18 000, 12 000 and 9000, the difference between the first two may be reckoned and that could be compared with the interval between the second and third values. Such comparisons are not possible with categorical data (in groups), nominal data (by types, e.g. blonde, brunette) and ordinal data, which are ranked by importance but given no absolute value. Parametric tests should be used on interval data.