Schedule Performance Index ( SPI) (ratio)
A Schedule Performance Index ( SPI) threshold value is expressed as a ratio. An issue is generated if the Schedule Performance Index (the ratio of the earned value of work performed to the cost of work that was scheduled) falls beyond the threshold values.
The Schedule Performance Index is calculated as SPI = Earned Value Cost / Planned Value Cost. A value less than 1 indicates that less work was actually performed than was scheduled.
If the threshold value is 1, an issue is generated whenever the value of the work performed falls below the expected cost of performing that work, according to the schedule.