The indicator which can tell more than other indices can tell.
What is a schedule performance index?
It is an indicator which measures the earned value against the planned value at a certain point along the project time and it is usually measured by the accumulated values reached up to the said point.
Schedule Performance Index (SPI) = Earned Value (EV) / Planned Value (PV)
If the SPI > or = 1 then the project ahead the schedule
If the SPI < 1 then the project is behind the schedule
How to let the schedule Performance Index tells you the story of a project?
by collecting the value of the SPI for each time unit that is passed, whether this time unit is a day, week, month, or even quarter, then put them in a line-chart and the result shall tell you the time when the project was a head and the time when the project was behind the schedule.
this analysis is a major key to know what is the factors which affected the project delays, and the factors which enhanced the project progress along the project's elapsed duration