The Schedule Performance Index is usually analyzed in conjunction with the Cost Performance Index (CPI), which tells you how far over or under budget, as opposed to ahead or behind schedule, the project is.
SPI and CPI are both greater than 1.0: The project is ahead of schedule and under budget (hooray!)
SPI is greater than 1.0 and CPI is less than 1.0: The project is ahead of schedule but over budget. In other words, more tasks have been performed than were scheduled at this point, but the tasks that have been performed are over budget.
SPI is less than 1.0 and CPI is greater than 1.0: The project is under budget but behind schedule. In other words, less tasks have been performed than were scheduled at this point, but the tasks that have been performed are under budget.
SPI and CPI are both less than 1.0: The project is behind schedule and over budget (boo!)
The Schedule Variance (SV) is similar to SPI but tells you the absolute, rather than relative, amount the project is ahead or behind schedule.
