TOP SEVEN DFD COMMON MISTAKES

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TOP SEVEN DFD COMMON MISTAKES
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Mistake 1 Verb vs. Noun
1
.
0
.
Verify order
Order
Process labels should be verb phrases
Data flow labels should be noun phrases
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Incorrectly labeling processes or data flow
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Mistake 2 DFD ? Flowchart
DFDs are not Flowcharts
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Mistake 3 1 2 3

• Process are numbered within the diagram NOT
  according to the correct process flow but for
  understanding purpose ONLY.
• logical representations - modeling WHAT a system
  does, rather than physical models showing HOW it
  does the work

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Mistake 4 Complicated

• Including more than nine processes on a DFD
• Decomposition is needed
• The purpose of data flow diagrams is to provide a
  semantic bridge between USERS and systems
  developers.
• User understanding and reviewing

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Mistake 5 CD ? DFD
Order
WAREHOUSE
CUSTOMER
In
-
Stock
Request
Payment
0
Status
Shipping
Message
Order
Order
System
Invoice
Shipping Confirmation
Reports
Inventory
Context Diagram of Order System
Entity ?
2
Data flow ?
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Mistake 5 CD ? DFD (cont)
Order
In
-
Stock Request
DFD Level 0 of Order System
CUSTOMER
WAREHOUSE
1
.
0
Status
Message
Entity ?
3
Check
Status
Status Data
Order
2
.
0
Data flow ?
Data
6
Pending
Issue
D
1
Orders
Status
Messages
Payment
4
.
0
Order Data
Invoice
Manage
Accounts
Receivable
5
.
0
Accounts Receivable Data
Accounting Data
Produce
Reports
Accounts
D
2
Receivable
Inventory
Reports
ACCOUNTING
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Mistake 6 Diagramming rules
Miracles
Black holes
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Mistake 7 DFD ? System

• DFD should reflect/represent the system)

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Tests for Correctness

• Are there any missing data flows?
• Are there any extraneous data flows?
• Are there any meaningless labels on data flows?
• Are there any missing processes?
• Are there any errors in leveling?
• Are there any deceptive process labels?
• Is there any flow of control information?
• Is the DFD an accurate representation of reality?

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Tips and Guidelines

• System boundary establishment is an important
  judgment call. External entities aid in
  determining where the boundary is established. An
  interfacing system can be shown as an external
  entity. It may be necessary to dictate the input
  of the external entity to assure system control.
• Label your processes carefully and vividly. A
  process that is labeled "Produce Report" and has
  the output of "Report" tells a reviewer very
  little. If you have trouble labeling anything on
  the diagram, it often is because you do not have
  adequate understanding. Choose names carefully.
• Think logical, not physical. Ignore media, color,
  font, layout, packaging, time, sequencing,
  etc. Think "what", not "how". Something
  logical can be implemented physically in
  more than one way. Including "when" and
  "where and "how" means you are getting
  physical.

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Tips and Guidelines (cont)

• Think data, not control flow. Data flows are
  pathways for data. Think about what data is
  needed to perform a process or update a data
  store. A data flow diagram is not a flowchart and
  should not have loops or transfer of control.
  Think about the data flows, data processes, and
  data storage that are needed to move a data
  structure through a system.
• Concentrate first on what happens to a "good"
  transaction. Systems people have a tendency to
  lose sight of the forest because they are so busy
  concentrating on the branches of the trees.
• Reviewers will not be convinced by confusion. A
  quality data flow diagram will be so simple and
  straightforward that people will wonder what took
  you so long.

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Tips and Guidelines (cont)

• Data store to data store, external entity to
  external entity, or external entity to data store
  connection usually do not make sense. Data flows
  with an arrowhead on each end cause confusion in
  labeling. Do not use them.
• Do not try to put everything you know on the data
  flow diagram. The diagram should serve as index
  and outline. The index/outline will be "fleshed
  out" in the data dictionary, data structure
  diagrams, and procedure specification techniques.

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The endThank you!
Ainee SAD Team (2007)