The realized cost of the work performed during a period. AC is what you spent, whether or not the spend bought the progress you planned.
Mnemonic. AC is the cash that left, not the progress it bought.
Exam trap. A low AC is not automatically good. Pair it with EV. Cheap and unfinished is a small AC with a smaller EV, which is still a poor CPI if EV is lower.
SPI equals earned value divided by planned value. An SPI below 1 means less work has been accomplished than the plan called for by this date.
Mnemonic. SPI is EV over PV. Under 1 is behind the scheduled work.
Exam trap. SPI is not a measure of whether people are busy. A team can be fully booked and still have an SPI below 1 if the finished scope is short of the plan.
BAC is the total approved budget for the work. It is the sum of the planned value for the whole scope and does not change unless the cost baseline changes.
Mnemonic. BAC is the whole approved pie, not the slice you have eaten.
Exam trap. Do not replace BAC with the latest forecast. The forecast is EAC. BAC stays the baseline until a change is approved.
The expected total cost of the project when the defined work is done. If the current CPI is expected to continue, a typical formula is BAC divided by CPI.
Mnemonic. EAC is the new forecast of the whole bill. BAC divided by CPI is the version you use when the past cost rate will continue.
Exam trap. Other formulas exist for other assumptions, including actual cost plus the remaining budget. The trap is using BAC divided by CPI when the stem says the overrun was a one-time event that will not repeat.