If you must decrease a project’s duration, make sure that overall project quality is not unintentionally lowered. For example, testing and quality control often occur last in a software development project; if project duration is decreased late in the project, those tasks might be the ones to suffer with cutbacks. You must weigh the benefits of decreasing the project duration against the potential downside of a deliverable with poorer quality.
If the budget (cost) of your project decreases, you might need more time because you cannot pay for as many resources or for resources of the same efficiency. If you cannot increase the time, you might need to reduce project scope because fewer resources cannot complete all of the planned work in the time remaining.