Rob Arthur for FiveThirtyEight:

WAR looks like a single easy-to-understand stat, but it’s the product of a complex model. That model integrates information on all the ways a player provides value: his hitting, fielding, baserunning and (for pitchers) pitching. The WAR you find at, say, FanGraphs or is an estimate of all those categories combined.
However, like all statistical estimates, WAR calculations come with uncertainty. Here’s where things get pretty statsy, so bear with me: You’re about to get a crash course in confidence intervals. The true value of a player varies from what you find on the leaderboard — but we’re not sure by how much.

Disagreed. There is one single stat which should define the 2015 AL MVP race: the playoffs. And the Angels are on the outside looking in.