There has been an ongoing debate for the past few days about who the National League Rookie of the Year should be, with most agreeing that the player should be either Jason Heyward or Buster Posey. This year's rookie class is stacked, but these two are obviously the best candidates. In most years, either would win the award rather easily.
Here are their stats compared, with more in-depth statistics being mentioned towards the bottom.
BA: .285 .321
OBP: .400 .369
SLG: .472 .516
PA: 582 401
HR: 18 15
RBI: 71 62
R: 81 52
Basically, Posey has gotten more hits on average compared to Heyward, but Heyward has walked a ton more. Even though Posey has the substantially better average, Heyward's ability to get on base via the walk gives up the much higher on base percentage. On base percentage is probably the most important stand alone statistic, and Jason Heyward may become one of the very few players in the history of the game to record a .400 OBP at age 20.
The award is based off production this season. Although Posey has the higher slugging percentage, which is due in large part to having the higher batting average, he still has less overall extra base hits than Heyward. This is something that should be included, since the results are based on the full season and not just rate statistics.
Now, let's look at second half statistics. Most of the time, including last year when Chris Coghlan won the award, a late season or second half push can win a young player the award.
Second Half Stats:
BA: .322 .303
OBP: .437 .357
SLG: .491 .485
PA: 279 252
HR: 7 8
RBI: 26 37
R: 40 30
Again, Heyward is getting on base at an Earth-shattering pace. His .437 mark is highest amongst all National League players in the second half, and his .322 batting average is now much better than Posey's. Posey still has hit for more power, but not making an out is more important than putting yourself in scoring position. They bat in different spots of their lineups, which is the main factor for Posey having more RBI and Heyward having more runs.
With the season total and second half statistics being put forth, we can now look at the more advanced statistics that many voters won't properly attribute when they put in their ballots.
wOBA: .384 .378
fWAR: 4.6 3.9
WPA: 5.09 1.26
These stats show that they have had relatively similar seasons offensively, with Heyward being a bit more productive because of his ability to get on base and the fact that he has gotten some bigger hits, as evident by his WPA.
WPA is a statistic that credits each negative plate appearance and debits each positive plate appearance, with the score and time of the hit raising the potential value of each plate appearance. It is not a predictive statistic, but it basically says how clutch a player has been this season -- whether luck or performance is the reason is not included in this statistic.
wOBA is basically OPS with a higher stress on OBP. As I mentioned before, getting on base is more important than putting yourself in scoring position, which is why wOBA was created to begin with. According to wOBA, Heyward is having a slightly better season.
Posey plays the tougher position, but WAR includes his value behind the plate. In terms of Fangraphs WAR, Heyward is still having the better season, even with the extra boost Posey gets for playing catcher. The same can be said with Rally WAR, where Heyward also has a sizable advantage. Heyward's tremendous defense and solid base running are skills aren't seen in his rate statistics, but they are accounted for in WAR.
According to Dewan's +/-, one of the top fielding statistics out there, Heyward has been the best defensive right fielder in baseball. Better than Ichiro, Werth, and every other right fielder in the game. This is especially impressive when you consider that there are many right fielders who are actually centerfielders forced to play right because of the team they are on.
Overall, even when you include Posey's positional advantage, Heyward is still the clearcut choice in terms of rate statistics and counting stats. Posey has a bit of an argument with his power, but you can't deny Heyward's on base abilities.
If Jason had not suffered that wrist injury that caused his horrible June and put him on the disabled list in early July, we may be talking about Heyward as an Most Valuable Player candidate instead of just a Rookie of the Year candidate.