What is the Schedule Performance Index and what does it tell you?

The Schedule Performance Index, or SPI, measures schedule efficiency by dividing Earned Value by Planned Value. SPI equals EV divided by PV. An SPI of 1.0 means the project is exactly on schedule. An SPI greater than 1.0 means the project is ahead of schedule, completing more work than planned. An SPI less than 1.0 means the project is behind schedule, completing less work than planned. For example, an SPI of 0.8 means the project is only completing 80 percent of the work planned for this point in time.