Difference between Top down approach and bottom down approach

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The difference between the Top-down approach and bottom down approach is:-

 Top-down Approach

- Top-down estimating method is also called Macro Model. Using the top-down estimating method, overall cost estimation for the project is derived from the global properties of the software project, and then the project is partitioned into various low-level components.

 - This method is more applicable to early cost estimation when only global properties are known. In the early phase of software development, It is very useful because there is no detailed information available. This approach starts at a system level. The estimator starts by examining the overall functionality of the product and how that functionality is provided by interacting subsystems. 


Advantages

  • It focuses on system-level activities such as integration, documentation configuration management, etc, many of which may be ignored in other estimating methods and it will not miss the cost of system-level functions.
  • It requires minimal project detail, and it is usually faster, easier to implement

Disadvantages

  • It often does not identify difficult low-level problems that are likely to escalate costs and sometimes tends to overlook low-level components.
  • It provides no detailed basis for justifying decisions or estimates.



Bottom-up  Approach

Using the bottom-up estimating method, the cost of each software component is estimated and then combines the results to arrive at an estimated cost of the overall project. It aims at constructing the estimate of a system from the knowledge accumulated about the small software components and their interactions. The leading method using this approach is COCOMO's detailed model.


Advantages

  • It permits the software group to handle an estimate in an almost traditional fashion and to handle estimate components for which the group has a feel.
  • It is more stable because the estimation errors in the various components have a chance to balance out.


Disadvantages

  • It may overlook many of the system-level costs (integration, configuration management, quality assurance, etc.) associated with software development,
  • It may be inaccurate because the necessary information may not available in the early phase.



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