A case is the primary unit of data in the data file. A case usually corresponds to a questionnaire.
Some complex applications have a hierarchical set of questionnaires. For example, the main questionnaire may consist of a household roster and other household information, and there may be a separate questionnaire for each woman in the household. The data entry application may then contain two levels: one for the household and one for each woman in the household. The set of forms corresponding to the household make up level one. The set of forms corresponding to each woman make up level two. Each case would consist of a level one and a variable number of level occurrences for level two. Most applications consist of a single level.