There are two common errors when forecasting the final cost of a project. One is to compare actual cost with planned cost. The other is to compare actual progress with planned progress. Both are wrong.
Earned Value teaches us that the only valid measure is to compare actual cost with actual progress. This may seem a bit like “comparing apples with oranges” – we seem to be comparing things that are not the same. The trick is, before we compare them, we convert them both to the same numerical units. That’s what makes the comparison possible and enables all the predictive goodness of EVM.
(Thanks to Glen Alleman for inspiring this brief post, with his comments here).