Software projects are subject to a multitude of problems that lead to schedule and cost overruns and poor quality of delivered software. Studies have repeatedly shown that a majority of failures in software projects are due to poor definition of requirements, uncontrolled changes to requirements and unreasonable estimates.software requirements and estimation provides software professionals the information they need to address requirements engineering and estimation. Both these activities are carried out at the start of the project and are the root cause for most of the problems that projects face. Requirements form the basis of the initial estimates and plans on which the software product is built and validated. Projects that are planned and executed using unrealistic estimates start-off on the wrong foot. The project manager and others who attempt to complete work within unrealistic cost budget, manpower budgets and schedules, either drive themselves crazy or take shortcuts that turn out very expensive later. Either way, things do not work out as satisfactorily as one would like them to. - from Amzon