‘Classification’ means arranging things in an appropriate order and putting then into homogeneous groups for example, in library, the books and periodicals are classified and arranged according to subjects. Students are grouped according to division they secure in a certain examination.
Plants and animals may be assigned to groups distinguished by structure, origin etc. Data may be arranged by time or space or both.
For example, we have time series data on aggregate income, aggregate consumption, size of the population
