Earned Value / Metrics
SPI (Schedule Performance Index)
SPI = EV / PV (Earned Value divided by Planned Value) -- a cost-based measure of schedule performance, above 1.0 meaning ahead of schedule and below 1.0 meaning behind, that is not the same thing as a CPM-based metric like CPLI or float.
Definition
SPI compares the budgeted value of work actually completed (EV/BCWP) against the budgeted value of work that was planned to be complete by now (PV/BCWS). It's a genuinely useful, standardized measure of dollar-weighted progress, but it knows nothing about logic, dependencies, or the critical path -- a project can show a healthy SPI while a real critical-path problem exists underneath it, because cheap, non-critical work finishing on time can offset expensive critical-path work running behind, all inside the same aggregate dollar figure. SPI and network-based schedule health metrics like CPLI answer genuinely different questions and shouldn't be conflated.
Example
A project shows SPI = 450,000 / 500,000 = 0.90 (behind schedule in cost terms) -- a real signal, but one that says nothing on its own about whether the critical path itself is actually healthy or already in trouble.