Hierarchical data is data that can be categorized at various levels. In a survey of households you might collect information about the household in general (for example, area of residence, type of residence, social class) followed by data about each individual in that household (for example, age, sex, occupation). So you have a hierarchy with the household data at the top, and the groups of personal data underneath.

Data of this type is frequently entered using trailer cards for the information which occurs more than once; in this example, the answers given by each person in the household. This is not always the case. Very occasionally you will come across surveys in which all data has been entered as one long record; this section describes how to deal with both types of hierarchical data.