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chaPter 7 • Using Data Flow Diagrams 183

Drawing Diagram 0 (The Next Level)
More detail than the context diagram permits is achievable by “exploding the diagrams.” Inputs and
outputs specified in the first diagram remain constant in all subsequent diagrams. The rest of the
original diagram, however, is exploded into close-ups involving three to nine processes and showing
data stores and new lower-level data flows. The effect is that of taking a magnifying glass to view
the original data flow diagram. Each exploded diagram should use only a single sheet of paper. By
exploding DFDs into subprocesses, the systems analyst begins to fill in the details about data move-
ment. The handling of exceptions is ignored for the first two or three levels of data flow diagramming.
Diagram 0 is the explosion of the context diagram and may include up to nine processes.
Including more processes at this level will result in a cluttered diagram that is difficult to under-
stand. Each process is numbered with an integer, generally starting from the upper left-hand
corner of the diagram and working toward the lower right-hand corner. The major data stores
of the system (representing master files) and all external entities are included on Diagram 0.
Figure 7.3 schematically illustrates both the context diagram and Diagram 0.
Because a data flow diagram is two-dimensional (rather than linear), you may start at any
point and work forward or backward through the diagram. If you are unsure of what you would
include at any point, take a different external entity, process, or data store, and then start drawing
the flow from it. You may:
1. Start with the data flow from an entity on the input side. Ask questions such as: “What hap-
pens to the data entering the system?” “Is it stored?” “Is it input for several processes?”

Figure 7.3
Context diagrams (above) can
Entity be “exploded” into Diagram 0
1 Input A 0 (below). Note the greater detail in
Diagram 0.
Output C
System Entity
Input B Name 3
Entity
2

1 2

General
General
Entity Input A Process Data Flow B Process Output C Entity
1 AAA BBB 3
Data Flow C

Record A Record E

D1 Data Store 1 D2 Data Store 2

Record A Record E

3 4

Entity Input B General Data Flow D General
2 Process Process
CCC DDD