During the project， requirements change for a variety of reasons.As needs change and as work proceeds， additional requirements are derived and changes may have to be made to the existing requirements.It is essential to manage  these additions and changes efficiently and effectively.To effectively analyze the impact of the changes， it is necessary that the source of each requirement is  known and  the rationale  for any change is  documented.The  project manager  may，  however，  want  to  track  appropriate  measures  of  requirements volatility to judge whether new or revised （） are necessary.