The Schedule Performance Index (SPI) is similar to the CPI.
It is used to estimate the projected time to complete the project.
A schedule performance index of one or 100% means the project is on schedule.
A schedule performance index greater than one or higher than 100% means the project is ahead of schedule.
A schedule performance index of less than one or less than 100% means the project is behind schedule.
Negative values on the performance indexes mean the project is either running out of money faster than planned or will take longer than planned.