Cost Incurred versus Work Actually Accomplished
There are three elements: (1) Cost incurred to date and (2) Work actually completed, and (3) the schedule.
There may be little relationship between cost and results. That is, the project may be incurring costs but there may be few results to show for it.
Costs incurred are easy to measure; % complete (work accomplished) may be difficult to measure.
Refer to CII Pub 6-5. It is in 08 Project Controls on the Web