Parent-child data problem

The SAS code you wrote would eliminate the observation for subjectid=102. This is because the aeyn field is not populated for that row and is therefore eliminated by the WHERE clause in SAS. This is a classic parent-child data problem in clinical trial data, where the parent question is left unanswered but the child response is given. A way to handle this problem would be either to include the aetext field in the WHERE clause or to add a warning to the SAS log. The code in Program 1.4 does both. 

A network model is quite similar to but more general than the hierarchical model. In a hierarchy, data have to be arranged such that one child has only one parent, and in many instances this is unrealistic. If we force the use of a hierarchical representation in such cases, data will have to be repeated at more than one location in the hierarchical model. This redundancy can create a number of problems. A record in a network model can participate in several relationships. This leads to two primary structural differences between hierarchical and network models. Some fields in the hierarchical model will become relationships in a network model. Further, the relationships in a network model are explicit and may be bidirectional. The navigation problem in a network data model can become severe. Because search of the database can start at several places in the network, there is added complexity in searching, as well. 

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