The Schedule Performance index (or SPI) is the ratio of the budget for the work performed to the budget for the work planned.  The SPI reflects the relative amount the project is ahead of or behind schedule, sometimes referred to as the project’s schedule efficiency.

You can use the SPI to-date to project the schedule performance for the remainder of the task or even the project.  So, if for the first few weeks of the project, the figures show that you are consistently doing work 10 percent faster than planned, it’s worth considering whether this is localized or will affect all the work still to be done and the whole project will end up delivering early.