Relationships among multiple sources of input data exist when each source contains common data, either at the physical or logical level. For example, employee data and department data could be related through an employee ID variable that shares common values. Another data set could contain numeric sequence numbers whose partial values logically relate it to a separate data set by observation number. Once data relationships exist, they fall into one of four categories:
The categories are characterized by how observations relate among the data sets. All related data fall into one of these categories. You must be able to identify the existing relationships in your data ...