All classification methods depend on exhaustive and mutually exclusive classes. Exhaustive classes contain all values in a given data range, no values are omitted. Before classifying or grouping data, it is necessary to determine whether the data are qualitative or quantitative and the level of measurement. This is important to know so that if required, the proper analyses may be applied to the data set. Certain analyses can only be applied on particular types of data.
Qualitative data are data that are grouped in classes according to differences in type or quality.
Qualitative data have no numerical values attached. Nominal data comes under this category. Quantitative data are data that contain attributes indicating differences in amount and can be expressed as numerical values. 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 .