NASA indicates that the probability of achieving the IOC milestone for the Ares 1 and Orion vehicles by March 2015 is 65 percent--that is, its level of confidence about meeting that date is 65 percent, which the agency considers to be a reasonable level for purposes of program planning. (NASA estimates the feasibility of meeting such milestones by using standard probability analyses of its plans for development programs.) NASA's 65 percent figure takes into account the reduction in its fiscal year 2007 budget (relative to the Administration's request) of $577 million (in 2007 dollars), a change enacted in the Revised Continuing Appropriations Resolution, 2007 (Public Law 110-5). The agency has accommodated the cut in its 2007 funding by eliminating some future missions of its Lunar Precursor Robotic Program. (That program is designed to launch robotic spacecraft to the moon to collect data about the moon's surface to help plan future human lunar missions.)

This *summary* of a cbo analysis of where NASA stands with Constellation and finishing iss/shutting down sts is still quite dense but gives a fascinating glimpse into how these programs are managed.

CBO: An Analysis of NASA's Plans for Continuing Human Spaceflight After Retiring the Space Shuttle | SpaceRef - Space News as it Happens