5-3 Short Answer/Essay Questions: Data Flow Diagrams IT-510-X2879Advanced Info Technology 14TW2

1.
Data flow diagrams show the flow of data through systems and are used during problem analysis. Systems do not transform input to a desired output in a single step. DFD's capture the information about the transformations and the movement of data through a system with bubbles and arrows to show relationships (Freetutes.) One of the four common mistakes often made when using data flow diagrams is an illegal data flow. All flows begin or end at a processing step, in which data is changed into a form of the final output. This way, mistakes can be identified and followed back to their source in order to be fixed. That is why it is important to ensure that there are no diagramming mistakes. The following are four descriptions of common diagramming mistakes that should be kept in mind when using a data flow diagram: A source or a sink cannot provide data to another source or sink without some processing occurring Data cannot move directly from a source to a data store without being processed Data cannot move directly from a data store to a sink without being processed Data cannot move directly from one data store to another without being processed (Whitten;Bentley;Barlow, 360.)
Black holes, grey holes and miracles are a second class of mistakes when using data flow diagrams. These mistakes arise when the outputs from processing steps do not match their inputs. Black holes arise when a step has input flows but no output flows. Miracles are the complete opposite: an instance where a step has output flows but no input flows. Grey holes occur when processing steps have outputs that are greater than the sum of its inputs. A major mistake made when using data flow diagrams is treating them as flow charts. While flow charts show both processing steps and data transfer steps, DFD's only show essential processing steps. Flow charts may also include arrows without labels, but DFD's should never show an unnamed data flow since DFD's are meant to be used as a source of accurate documentation. Furthermore, while flow charts usually show different steps for the handling of each item of data, a data flow diagram can have multiple data items on a single flow arrow (Osborn, 1999.) Finally, processes should be numbered within a data flow diagram in an order that will make it easiest to understand, not necessarily in the correct process flow since the DFD will be used heavily in redesigns. It should give logical representations of the processes in order to model WHAT a system does opposed to physical models which show HOW it works. (Ainee, 2007.)

2.
A data dictionary is an important part of the system design documentation process when using data flow diagrams. Without it, a designer will only have a rough sketch of the system. Its function is to store detailed information about all of the elements that make up the total database of data stores and processes. With the development of CASE software tools, a data dictionary is called a data repository, containing all information pertaining to the interrelationships of data elements. All of this information is necessary for programmers to translate the diagrams into flow charts or directly into code. Since programmers rely on data dictionaries for the design of systems, they need to be accurate and thorough. Besides providing a reliable source of guidance throughout the development process for both experienced programmers and new members to a development team, data dictionaries are a source of historical documentation of the process meant as a safety measure in case the system ever needs to be redesigned (Sehlhorst, 2006.)

3.
Logical data models represent all of the functions or processes contained within an organization and are independent of any particular data management technology.. One of the major reasons that logical models are so useful in systems analysis and design is that designers are encouraged to think creatively because logical models do not take technical implementation issues into account. Since they are not forced to focus on the physical aspects, there is less risk of overlooking important system features. Due to the non-technical nature of logical models, they provide a useful method of communication between the systems analyst and end-users via simple, easily understood graphic tools. A data flow diagram is an example of a logical data model because it gives a logical representation of the processes in order to model what a system does. Physical models instead show representations of the processes in order to show how the model works. In short, logical data models are used to better understand a system for documentation purposes while physical models are literal diagrams of the system's processes. (Ainee, 2007.)
Works Cited

Ainee. Top Seven DFD Common Mistakes. Scribd. Published by user rahul. 2007. Accessed February 5, 2014. http://www.scribd.com/doc/26738942/DFD-Common-Mistake

Freetutes.com. Data Flow Diagrams (DFD). Accessed February 5, 2014. http://www.freetutes.com/systemanalysis/sa5-data-flow-diagrams.html

Osborn, Charles. Data Flows: Note on Data-Driven Process Modeling. 1999. Accessed February 5, 2014. http://faculty.babson.edu/dewire/Readings/dfdmistk.htm

Sehlhorst, Scott. Foundation Series: Data Dictionary Definition. July 13th, 2006. Accessed February 5, 2014. http://tynerblain.com/blog/2006/07/13/foundation-series-data-dictionary-definition/

Whitten, J; Bentley, L. D; Barlow, V.M. (1994) Systems Analysis and Design Methods (3rd ed.) Page 360. Burr Ridge, IL; Irwin.