Without looking up any definitions…
WAR attempts to capture a player’s total value relative to a mythical “replacement” player. There are slightly differing ways to determine how to calculate this, though everyone agrees on the general idea. Because of these differences, it’s important to make sure you’re comparing apples and apples. The two most well known and recognized WAR ratings are those calculated by Fangraphs (fWAR) and Baseball Reference (bWAR).
When we say “total value” for a position player, we are trying to include all of these things:
- Value from batting. How many runs has this player helped their team score?
- Value from defense. How many runs has this player helped their team prevent?
- Value from baserunning. How many runs has this player helped their team score on the basepaths, whether by from stolen bases, taking an extra base on a single, etc.
The numbers that go into the batting portion are the best numbers. More research has been done on hitting, and there is data available for a longer period of time. At this point, we have it pretty well down what helps you and what hurts you at the plate. Cabrera has had a very slightly better offensive season than Trout by most measurements. (Actually, after last night, fWAR gives Trout a very small offensive edge, but they’re functionally equal here, and both well clear of the rest of the AL.)
The numbers that go into the defense portion are a little bit fuzzier. In particular, the sample sizes are much smaller over time for fielding: 600+ PAs for hitting, far fewer opportunities in the field. We’re not look at errors here, either; we’re looking at stuff like “how many balls hit in his general direction did he field and turn into an out, relative to how many he should have?” It’s mostly agreed that single season defense numbers are good general indicators, but you shouldn’t get too worked up over specifics. There is also a positional adjustment baked into this number: a 3B is more valuable than a 1B, a SS is more valuable than either. Generally, Cabrera rates from a slightly below average to significantly below average 3B, while Trout is a significantly above average CF. Huge edge to Trout here.
The baserunning numbers are surprisingly good. Not necessarily as a predictor of future performance, but as a recording of what’s already been done. We know that stolen bases are X valuable, caught stealing takes away some Y of value, and runners take extra bases some number of times over the course of a season, which is worth Z. Cabrera, again, is slightly to somewhat below average here, while Trout is near the top of the league.
Once you add it all together, Trout ends up being 30 or 40 percent more valuable than Cabrera, largely on the basis of baserunning and defense. The only way you can even make the two of them close is by saying that these don’t matter at all, or that Cabrera is somehow comparable to Trout.