The numbers used to calculate these estimates can come from several sources.
They can be provided with the methodology that is used, taken from projects with
similar tasks and technologies, or provided by experienced developers. Generally
speaking, the numbers should be conservative. A good practice is to keep track of
the actual time and effort values during the SDLC so that numbers can be refined
along the way, and the next project can benefit from real data. One of the greatest
strengths of systems consulting firms is the past experience that they offer to a project;
they have estimates and methodologies that have been developed and honed
over time and applied to hundreds of projects.