Combine the set of dfd fragments into one diagram the

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Combine the set of DFD fragments into one diagram – the level 0 DFD . Look for overlaps (sources, sinks, datastores) and combine. Try to identify flow. Is there one trigger that is more common than others? One process that occurs earlier in execution than the others?
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17 NOTE: No direct data flows between 3,4,5 so not a fully integrated system yet. What happened to processes 1 & 2? Who knows?
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Create Level 1 (+n) Diagrams Level 1 DFD – lower-level DFDs for each process in the level 0 DFD. Including external entities in level 1 and lower DFDs can simplify the readability of DFDs. Consider this the “zoom” function to open up and look into the details of any given process.
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Create Level 1 (+n) Diagrams There is no simple answer to the “ ideal ” level of decomposition, because it depends on the complexity of the system or business process being modeled. In general, you decompose a process into a lower-level DFD whenever the process is sufficiently complex that additional decomposition can help explain the process. Rules of thumb: - Typically 3-7 processes for any given every DFD. - Decompose until you can provide a detailed description of the process in no more than 1 page of process descriptions.
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22 “Level 2 of Process 4.3
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Validating DFDs Two fundamental types of errors in DFDs: 1. Syntax errors – can be thought of as grammatical errors that violate the rules of the DFD language. 2. Semantic errors – can be thought of as misunderstandings by the analyst in collecting, analyzing, and reporting on the system. Syntax errors are easier to find and fix because there are clear rules that can be used to identify them. Most CASE tools have syntax checkers.
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Validating DFDs Semantic errors cause the most problems in system development. Three useful checks to help ensure that models are semantically correct: 1. to ensure that the model is an appropriate representation by asking the users to validate the model in a walk-through 2. to ensure consistent decomposition 3. to ensure that the terminology is consistent throughout the model
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Syntax Rules: Process Must have a unique identifier (number and label) Label should be a verb phrase (except for the context level 0-process which is a descriptive system title) Usually three words: verb, modifier, noun On a physical DFD, could be a complete sentence Must have at least one input Must have at least one output (transformed!) # Process
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28 1.0 Gather Data 2.0 Compile Statistics Demographic Data 3.0 Analyze Responses Survey Responses Final Report Survey Instrument Stat Report
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29 2.0 Visa Authorization 2.0 Total Records 2.0 QA Process Inspect Finished Products 1.0 Check Customer Credit 2.0 Sum Sales Records 3.0 BETTER BETTER BETTER Naming individual processes
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Syntax Rules: Data Store Must have a unique identifier (number and label) Label should be a noun phrase No data flows between two data stores; must have a process in between No data flows between a data store and a source or sink; must have a process in between Data are not transformed, only stored Should have at least one data flow to be “in scope” # Data Store
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