The earned value management system, aka EVMS, is built out of linear equations! OMG! When did this happen? One answer is more than 50 years ago, but another answer is that it happened during the same age that other models came into vogue that purport to represent reality with linear relationships.
Don't get me wrong--I am in the corner with every advocate of earned value...how else can you measure progress toward outcomes? Certainly not by measuring the consumption of input. Consumption only tells you that something is absorbing input as planned; it gives not a clue whether input is being properly transformed into value producing outcomes for the project beneficiaries. Only some form of EVMS will do that.
My rub is that project managers too often fall in the trap of believing the forecasts beget by linear equations. They seem unaware that linearity represents only the rational part of our actions; and whereas rationality is what we all say we want because rational also implies predictablilty, in point of fact [or, at least as supported by field observation and analysis by neuroscience] humans are incapable of true rationality. We require, or better to say our brain processes require, a dose of emotion [read: dopamine] to cause our rational brain to stop cycling through alternatives.
But, that is not to say the calculated EVMS trend lines are incorrect: to do the same and expect different is a fool's game. Thus, this discussion arrives at the most important mission vis a vis EVMS:
"The mission of project management is to defeat an unfavorable forecast [more often than not wrought by the testy linearity of equations] and produce outcomes of reasonable value that do not exceed the expected investment."
Those with IMS/IMP experience will find these words a bit too informal, but nevertheless, they pretty well represent the bottom line.
By the way, take a look at my blog item "Beware the people model" at to get another idea about linear models and human behavior.
