The biggest question in the baseball internet community right now is whether Miguel Cabrera or Mike Trout is more deserving of the MVP award. Most of the more mathematically-oriented fans are choosing Trout and some are being quite adamant about it. There are many different interpretations of what "most valuable" means and I'm not going to tell you that the sabermetric way is the only way or that voting for Cabrera is wrong. I'm also not going to claim that those two are the only viable candidates. However, I do want to explain the basic thought process that Trout supporters are using and why so many are certain that he deserves it.
The statistic most commonly cited by sabers is Wins Above Replacement (WAR). There are several different WAR statistics, all of which indicate that Trout has been worth more wins to the Angels than Cabrera has been worth to the Tigers. Here are some of them:
What confuses a lot of people is that separate statistics with the same name give different results. They also hear that these statistics are questionable because they include defense which is very difficult to measure. So, I'm going to explain the entire WAR thought process step by step. You may decide that you don't like the method and want to use your own line of reasoning, but you should be able to understand, after reading through this post, why so many people are claiming that Trout is the clear MVP.
The easiest thing to measure is hitting, so I'll start there. Looking at the standard rate statistics, Cabrera is batting .333/.398/.616 and Trout .327/.395/.556. Batting average and on-base percentage are very close, but Cabrera has the lead in slugging. Consequently, Cabrera leads in OPS (1.014 versus .951) and the more accurate wOBA without base stealing (.421 versus .403).
Cabrera has more playing time (641 plate appearances versus 580), so he should get credit for that. Two simple statistics which do that are Times On Base (351 vs. 284) and Total Bases (255 vs 229), the countimg versions of OBP and slugging respectively.
Now, we need a counting statistic which is parallel to OPS or wOBA. We can translate all of the hitting events (walks, hit batsmen, singles, doubles, triples, home runs, outs) into estimated runs using Batting Runs Above Replacement (BRAR). The Batting Runs theory is discussed here. Cabrera has 76 BRAR which means that he has contributed an estimated 76 runs to his team's total above what you would expect from a replacement player. Trout has 70 BRAR. So, Cabrera has contributed more with his bat than Trout. We can say that Miguel has the Most Valuable Bat.
We can't stop there though, because there are other ways players contribute to their team beyond hitting. First, there is base running which is one of Trout's specialties. According to Baseball Prospectus Base Running Runs (BRR), Trout has contributed 10 runs with his base running compared to an average player. On the other hand, Cabrera has cost the Tigers -5 BRR. The BRR metric includes stolen bases, caught stealing, advancing from first to third on a single, advancing on a fly ball, etc.
Adding batting and base running (BRAR + BRR) yields 71 Runs Above Replacement (RAR) for Cabrera and 80 for Trout. So, Trout takes the lead.
Another thing to consider is ballpark effects. Based on five years of data, Comerica Park is slightly more hitter friendly than Angels park, so subtract one run from Cabrera and add two for Trout. That's hardly worth doing in this case, but I wanted to explain the entire process. With more extreme parks it would matter more. The new RAR totals are 70 for Cabrera and 82 for Trout. So, while Cabrera has added more runs with his hitting, Trout has added more runs with overall offense. We could say that Trout is the Most Valuable Offensive Player.
Finally, fielding needs to be considered. Third base and center field are considered to be equally important positions. So we don't really need to adjust for position, but I will anyway just to illustrate the general process. Cabrera gets a little edge for position because Trout has played quite a bit of left field which is less valuable than playing either third or center. Baseball-Reference gives Cabrera one run for his position and subtracts one for Trout. There would be a bigger impact if we were comparing a shortstop to a first baseman for example.
The trickiest part of the WAR process is determining how well they play their positions. You could punt and assume that they are both average fielders. That is not necessarily unreasonable if you are uncomfortable with defensive statistics. However, I don't think too many people would argue that Cabrera and Trout are equals defensively.
Tigers fans gave Cabrera an average rating of 49 for his defense on the Fan Scouting Report. That puts him in the middle of the pack (maybe a little below average) for third basemen. Angels fans gave Trout an average rating of 74 which is near the top of all center fielders. So, a bunch of Angels fans that watch a lot of games think a great deal more highly of Trout's defense than an equally avid group of Tigers fans think of Cabrera's defense. That is far from definitive, but I believe it tells us something about their defensive skills. If they voted similarly on both players, it might be fair to ignore defense, but in this case I think it's prudent to give Trout credit for his fielding.
There are several metrics which are designed to determine how many runs each player saved/cost his team. Here are some which have been discussed here previously:
Assume Average 0 runs
Total Zone -9
Assume Average 0
Total Zone +13
If we take the average of the five metrics for each player, we get -5 for Cabrera and +11 for Trout. Now, add these to the positional runs mentioned above, we get -4 Defensive Runs for Cabrera and +10 for Trout. So, Trout is the more valuable defender.
Finally, add the Offensive Runs to the Defensive Runs: 71-5 = 66 for Cabrera and 82 + 10 = 92 for Trout.
If you like to see the numbers expressed as Wins instead of Runs, you can divide by 10 giving 6.6 WAR for Cabrera versus 9.2 WAR for Trout.
That's another set of WAR numbers! That's not a problem though. The idea was to show the basic process of arriving at WAR. Different analysts plug in different numbers especially for fielding, but Trout always wins the WAR.