1 Chapter Data-Flow Diagrams Table of Contents

Discussion of Review Question 22
It should be remembered that each modelling technique (such as Data-Flow Diagrams and Entity-
Relationship Diagrams) only presents one aspect of the system — the model of the complete system
is formed when a number of different models are put together.
The three main traditional modelling techniques for systems analysis and specification are:
• Data-Flow diagrams
• Entity Relationship diagrams
• Entity Life histories
The third of these, Entity Life Histories (ELHs), is the modelling technique that represents those
aspects of a system that change over time. Entity Life Histories are introduced in a later unit, and play
an important role in relating the processes and data stores of DFDs with the logical data models of
Entity Relationship Diagrams.
Any particular modelling technique will have been designed to represent only certain aspects of a
system, since any non-trivial system would be much to complex to ever model with a single technique
— any resulting diagram or set of diagrams would contain more information that would be usefully
understandable by users and system developers.