Figure 13 The progression of DFDs

Figure 13 The progression of DFDs

In practice, the first one/two stages are often omitted and the DFDs begin with a construction of the required logical DFD. The logical DFD always appear to be simpler and usually show fewer processes than the corresponding physical DFD.

We will not describe the construction of physical DFDs here but you may like to look at some examples in Kendall and Kendall (1998) or Goodland and Slater (1995)

5. An example of the development of a Data Flow Diagram

In order to show the development of a logical data flow diagram we first need to have gathered together information as a result of fact finding. The techniques of fact finding include:

• 
  Observation
  
  • 
    Observing an individual in the environment in which they normally operate
    
• 
  Interviews
  
  • 
    Having a succession of interviews with representatives of the users
    
• 
  Questionnaires
  
  • 
    Gaining an overview of the required system and problem areas to be resolved
    

Having gathered together information relating to the system to be built we may represent it in a textual description, as given below.