In a context diagram, a DFD shows one process representing the entire system, all external entities, and all relevant data flows. A level zero diagram shows 5 +/- 2 processes at the same level of abstraction, all external entities, all data stores used by the processes, and all relevant data flows. DFDs are decomposed until each process is a functional primitive, and inputs/outputs must be balanced between levels. Logical DFDs do not show system behavior, timing, control, or specific media, and should avoid "black holes" where data disappears or "miracles" where processes create output without input.

1. Data Flow Diagram Concepts
 In a Context Diagram
 One process representing the entire
system
 All external entities
 All relevant data flows

2. Data Flow Diagram Concepts
 In a Level Zero Diagram
 Expect 5 +/- 2 processes
 (Attempt to have all processes at same
level of abstraction)
 All external entities
 All data stores used by these processes
 All relevant data flows

3. Data Flow Diagram Concepts
 Decomposition
 Decompose until process is clear enough
that no further explanation is needed
(functional primitive).

4. DFD Concepts
 Balancing
 Inputs and outputs from higher level
process must be reflected in lower-level
(decomposed) processes. These may be
described in greater detail in the lower-
level diagram.

5. Data Flow Diagram Concepts
 What is NOT in a LOGICAL DFD
 dynamics of system behavior
 timing
 control
 particular input/output/storage media

10. Data Flow Diagram Concepts
 Watch out for
 Black Holes
 data goes into a process (or data store)
but doesn’t come out
 Miracles
 a process produces output with no
input