Included in this category are ordinal , interval , and ratio . In some literature and statistical computer programs, no distinction is made between interval and ratio data, calling them both continuous . For example, a line could represent either a road or river, and a land use polygon could be residential, commercial, or a recreational area. Nominal data are often labelled with numbers or letters, but these labels do not imply ranking . A nominal datum can only be examined for its physical similarity to, or its difference from, other occurrences, or for the frequency of its occurrence.
Ordinal data provide information about rank or hierarchy, in other words, relative values.
Therefore, it is possible to describe one item as larger or smaller than another, or as low, medium, or high. However, it is not possible to measure the differences between ordinal data, because there are no specific numerical values attached to them. An example of ordinal data is roads ranked as expressway, main thoroughfare, and secondary road. Interval data , in addition to being ranked, include numerical values. The information can be arranged along a scale using a standard unit.