Use Case Diagram is UML’s solution for gathering requirements. Though Requirements Tree Diagram has certain constructs similar to Use Case Diagram and the main aim of both of them is to capture requirements, there are certain differences between them as follows:
- Requirements Tree Diagram follows a unique tree structure from top level requirements to most specific requirements enabling it to provide a facility of requirements traceability. Use Case Diagram mainly captures use cases, association between use cases and with actors. However Use Case Diagram fails to provide traceability for requirements or for use cases.
- Requirements Tree Diagram has enriched set of relationship between requirements. As compared to Use Case Diagram, Requirements Tree Diagram has Composition as additional relationship between requirements. Composition is the most widely used relationship in upper part of Requirements Tree Diagram. Differentiating Composition relationship from Includes relationship, Requirements Tree Diagram provides more clarity in requirements.
- Requirements Tree Diagram encourages capturing inputs, processing logic and desired output for all the requirements. With higher level of clarity in requirements, Requirements Tree Diagram provides useful inputs in accurately estimating the efforts required for implementation of requirements. However Use Case Diagram are seldom used as inputs for estimation.
- It is not necessary to capture all the use cases in Use Case Diagram. Rather all the use cases are never captured. Thus there is always a possibility of ambiguous requirements not captured thoroughly. However Requirements Tree Diagram recommends capturing all the requirements in depth avoiding ambiguous requirements.
- Use Case Diagram itself hardly provides any information about the use cases involved. Each use case mentioned in Use Case Diagram has to be supported with huge information like description of use case, actors involved in it, preconditions of the use case, main flow, alternative flow and many other details. Capturing all such information for every use case makes the document bulky. This also increases the complexity in understanding the use case. However in case of Requirements Tree Diagram, each requirement is supported with list of inputs, list of desired outputs and the processing logic limited to that requirement which is again recommended to be captured using flow chart diagram. Thus Requirements Tree Diagram avoids bulky paragraphs putting most of the information in diagram format only.
- Use Case Diagram (or even use case) do not capture all the input parameters in depth. However Requirements Tree Diagram recommends capturing minute details for every input element which may be a user screen, database or even a XML file. Such details can be captured in spreadsheets (and of course not in paragraphs.). The level at which such details to be captured is always a user’s choice.
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