Chapter 13Estimating Software Requirements
Software requirements are the starting point for every new project, and are a key contributor to enhancement projects, as well. Software requirements are also very ambiguous, often filled with bad assumptions and severe errors, and are unusually difficult to pin down in a clear and comprehensive way.
From a software cost-estimating standpoint, the most tricky part of estimating requirements is the fact that requirements are usually unstable and grow steadily during the software development cycle in the coding and even the testing phases.
The observed rate at which requirements change after their initial definition runs between 1 percent and more than 3 percent per month during the subsequent analysis, ...