Why Impact Analysis?

Impact analysis can be used as a basis for estimating the cost associated with a change which in turn can be used to decide whether or not to implement it depending on its cost/benefit ratio. Impact analysis forms an important part of project management since changes to software often are initiated by changes to the requirements.

Source: Engineering and Managing Software Requirements (Aybüke Aurum · Claes Wohlin Pg. 118)

Changes are inevitable in a project environment but accommodating and analysing the impact at an earlier stage makes it cheaper. According to Bohner and Arnold requirements change from the point in time when they are captured until the system has been rendered obsolete. During design, code does not yet exist, so new and changing requirements affect only existing requirements and design. A different study (by Boehm and Papaccio) also identified that it costs US$1 to locate and fix an error in the requirements definition stage, US$5 in the design phase, US$10 in the coding phase, US$20 during unit testing, and up to US$200 after system delivery (Aybüke Aurum · Claes Wohlin Pg. 2).