Hi:

Well the authors of the BABOK 2.0 were wise enough to recognize the asychronous nature of systems:  That many tasks\processes can be occuring at the same time or that they can often occur in any possible sequence.   That is why they used input\process\output diagrams to organize the BABOK 2.0. 

(In case you have not noticed: The BABOK 2.0 is largely a functional spec on how to create a functional spec.   The "system" being spec'd is the series of interrelated tasks that a BA typically performs in do his/her work.)

Data flow diagrams are simply integrated input\process\output diagrams.  Soooo, as the BABOK authors lay claim to setting our certification criteria, and as actions speak louder than words, let it henceforth be know to all in BA lands everywhere that DFDs are no longer a "dated" or old technique, but that they are once again the latest and greatest (read: hip, modern, cool, neato, etc, etc).  

Tony Markos

Use a DFD because it is describe how a data should flow and how it’s related to Entities and Processes. You can use a flowchart when you want to explain how things suppose to work.

Steve:

You can use only flow charts for smallish efforts.  But if you have a complex system - and business systems tend to be complex - you need decomposition from higher levels of abstaction to the more detailed level.   Problem:  At the bigger picture level, more complex systems are not sequential, and therefore sequence based flow charts can not be used to model them.    Once one decomposes downward to detailed levels, then, one switches over to sequence based flow charts to handle the detailed logic.

The above are the basics of DFD's, although sooooo very few now the real basics of DFDs.

Latest update:  Some "people in the know" have recently told me that BPMN, a sequence based modeling technique, can handle deep decomposition.   They told me such is explained in a book (that I have on order).  If the BPMN does in fact handle deep decomposition, then DFD's are obsolete, as DFD's are harder to create.

But, if BPMN does not handle deep decomposition, then, the BA community has a problem:   People who do not have the work experience to often  coming across as they do.

Tony