Common mistakes in DFD’s…

7 Mar

In the blog I will discuss some of the commonly made mistakes in regards of Data Flow Diagrams.

Illegal Data Flows

  • These are mistakes that are easy to check for and easily corrected.
  • One of the patterns of data flow analysis is that all flows must begin with or end at a processing step because obviously data cannot simply metastasize on its own without being processed
  • This simple rule means that the following mistakes can be fairly easily identified and corrected in a DFD. Diagram
  • Diagram below to explain:

mistakes IS

Diagramming mistakes

A second class of DFD’s mistakes result from when the outputs from one processing step do not match its inputs. For example:

  • A processing step may have input flows but no output flows. This situation is sometimes called a black hole.
  • A processing step may have output flows but now input flows. This situation is sometimes called a miracle.
  • A processing step may have outputs that are greater than the sum of its inputs – e.g., its inputs could not produce the output shown. This situation is sometimes referred to as a grey hole.


DFDs are not flow charts

This mistake however is more difficult to identify then the two previous.

  • Flow chart diagrams can be useful for describing programming logic or understanding a single sequence of process activities.
  • Flow charts often show both processing steps and data “transfer” steps, DFDs only show “essential” processing steps.
  • Flow charts might include arrows without labels: DFDs never show an unnamed data flow.
  • Flow charts show conditional logic, DFDs don’t.
  • Flow charts show different steps for handling each item of data, DFDs might include several data items on a single flow arrow.




3 Responses to “Common mistakes in DFD’s…”

  1. sad112540853 March 8, 2013 at 12:38 am #

    i think i now know the difference between data flow diagrams and flow charts, thanks 😀 this blog was very helpful as i thought they were the same thing 😀

  2. sad111708665 March 8, 2013 at 7:35 pm #

    ….excellent information about Data Flow Diagrams….should be useful for the exam…thanks for your blog…

  3. sad111468572 March 10, 2013 at 12:29 am #

    great stuff 🙂

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