Will MLB's tremendous rookie class of 2015 suffer a sophomore slump?
The 2015 season was an unquestionably special one in baseball: We witnessed teams that hadn't made the postseason in 20 years break through the barrier, saw latent fan bases reborn and, finally, after falling just short in 2014, a magic-fueled, unconventional team won the World Series.
Because of a mixture of player development trends and incredible fortune, we also saw something exceedingly rare: the best positional rookie class in the past century. The likes of Kris Bryant, Carlos Correa, Matt Duffy and many others announced their arrivals with exceptional first-year performances, taking the league by storm and bringing about a new golden age for young, ultra-talented position players.
We've stated many times how much better the 2015 rookie positional class was than any other in the past 100 years. The fact bears repeating again because of its magnitude. Below you'll find a chart showing the average Wins Above Replacement for positional rookies for a full season of plate appearances (600 plate appearances) from 1920 to 2015:
As we can see, no other season is close to 2015. There are a few reasons for that, reasons that are complex and outside the particular scope of this article. For now, this should serve as background for what we're discussing today: how sophomore/follow-up seasons compare to great rookie campaigns. We'll be focusing specifically on the years that followed rookie-of-the-year campaigns to try to discern whether a "sophomore slump" occurred among the league's best first-year players, and we'll also bring in the 2016 projections for our exceptional crop of 2015 rookies to look at next year's expected performance.
Overall_Rookie WAR:600, 1920-2015_1
Let's start with our rookies of the year. I've pulled every rookie of the year winner since the award was first given out in 1947, compiling data on their first and second seasons. First, let's understand how rookie-of-the-year production has changed over time, as well as acquaint ourselves with some notable examples. Here is a chart of how many WAR each award winner (excluding pitchers) accounted for in their winning season, with labels on rookies who produced greater than 7.0 WAR:
There are few opportunities to use the word legendary to accurately describe an event in baseball, but this is one of those times: Mike Trout's rookie season was truly legendary, and it's nice to be regularly reminded of that fact. Interestingly, the position players selected for rookie of the year honors have, on average, increasingly gotten better over time, with an average improvement of about one win from the beginning to the end of the time frame. While there is a lot of variability among the winners, we haven't seen a winner with fewer than 2.0 WAR in at least 15 years (Ben Grieve, 1998). This makes sense, as we can see in our first graph that the average rookie of the past 15 years is consistently more productive than the average rookie that came in the decades before them.
Rookie of the Year WAR (Per 600 PAs), 1947-2015
Now let's look at the production of rookie of the year winners in the season following their award, again adjusted to 600 plate appearances to smooth out any playing time discrepancies. Did many of our rookie of the year winners suffer a sophomore slump in their second year in the league? Here's the difference in WAR over 600 PAs, with blue as more productive during the second year in the league and red as less productive:
What we have here is pretty stark, and it tells us something clearly: Most position players who won the Rookie of the Year Award performed worse during their second year in the league. Out of all position players who have won the award since 1947 -- 99 of them, to be exact -- only 32 performed better during their second year. If we lower the bar slightly -- allowing sophomore performance to be one win below rookie performance -- we only increase our tally of players to 55. In absolute terms, the average change in performance for positional rookie of the year winners from their first to their second year was almost an entire win, at -0.9 WAR. For a full breakdown of the each player, you can find the data here.
WAR:600 Difference, Season After RoY, 1947-2015
This shouldn't come as too much of a surprise, as most award winners -- whether rookie or not -- are, by their very nature, outside of a "normal" performance level. As with the MVP and Cy Young, many (and possibly most) players who end up winning these awards do so on the back of one outstanding, aberrant season. It is the rare player who can consistently compete for awards, and we end up knowing their names by heart: Mays, Ripken, Bagwell, Trout. It's not a surprise that many players who win rookie of the year go on to win other awards, but it's also not a surprise that most fail to do so. To win one major award in baseball is an incredible achievement; to win many usually means a chance at the Hall of Fame.
Finally, that brings us to this year's rookie winners, Kris Bryant and Carlos Correa, and their projections for the 2016 season. Many fans might be hoping for even more production out of these two players than they saw in 2015, and that is a completely natural thought: With rare talent at such a young age, we can be forgiven for thinking that the best is yet to come. And while we might be proven right in thinking that over the long term, the projections for next year tell us we should temper our immediate expectations. Take a look at Bryant and Correa's 2015 WAR/600 alongside their 2016 projections, courtesy of Steamer:
It's not a coincidence that Bryant is projected for exactly -0.9 fewer WAR than this past season, the average dip in performance for the year following a Rookie of the Year Award we found in our data above. If Correa had debuted earlier than June, he too would be right around -0.9. This isn't an agreed-upon figure between Steamer and myself; it's simply what we both arrived at through previous observations. Projections can be wrong -- they often are. We look at Bryant and Correa and see the shimmering future of two franchises. The immediate success they enjoyed isn't guaranteed, however. Opposing teams assemble scouting reports in the offseason, pitchers study tape, and some adjustments have already been made on Opening Day.
| 2015 WAR/600 | 2016 Projected WAR/600 (Steamer) | WAR/600 Difference | |
| Kris Bryant | 6.0 | 5.1 | -0.9 |
| Carlos Correa | 4.6 | 3.9 | -0.7 |
The 2015 rookie class was possibly the best ever, and there's a reason for that. The confluence of player development trends, talent and great timing created a perfect storm of future franchise cornerstones. There's an unfortunate reality for the sophomores of 2016, however: Everyone else in the major leagues is really good, too.
