The Historic Excellence of 2015’s Positional Rookies

Last Monday, at Just A Bit Outside, I looked into this year’s positional rookie class, and the fact that they were the best first-half class in the past ten years when measured by Wins Above Replacement. They have more overall total WAR, more individuals with over 1.0 WAR, and represent the first rookie class in at least a decade to have two rookies with over 3.0 WAR in the first half (Kris Bryant and Joc Pederson).

In short, this is a sort of renaissance year for positional rookies, and the article led me to wonder just how great 2015’s rookies are when compared to a larger sample of years, and a larger number of criteria. So here we are!

We’re going to be focusing mainly on positional players in today’s piece, as they’re really the standout group this season; rookie pitchers are having an about average year (by WAR) compared to years in this past decade, so that’s a topic for another day. As a primer, I’ll provide two charts from last week’s piece to get the ball rolling, and to get us up to speed with what was already covered.

First, I took the top-20 rookies by first-half WAR for each season in the past decade and looked at combined WAR:

Then I simply charted how many rookies had above 1.0 WAR by the end of the first half of those seasons:

That provides a good base for understanding the performance of the positional rookies this year. It’s a top-heavy, strong class — in fact, it’s much stronger than any other positional class in the past decade. That’s a big deal, and it also begs the question of just how good this group is when compared to past decades, beyond the previous 2005 cutoff. As we’ll see, it also underlines a few long-term trends that pertain to how rookies are handled, when they arrive in the majors, and how they perform when they do.

Our FanGraphs leaderboards have first-half of the season splits going back to 1974, so I’ve pulled all of the data for rookies going back to that year: that gives us a nice round 40 years of data to work with. With that data, let’s simply look at the top-heavy nature of the 2015 rookies in the context of the longer timeframe.

We’ll recreate our first graph, charting overall combined WAR for the top-20 first-half rookies for each season going back to 1974. That should give us a better baseline idea of rookie production in different eras, and where this season fits into that history. (Note: all of the following charts are interactive, so you can hover over the bars to see how much WAR the top 20 rookies of each year produced, as well as how many rookies posted at least 1.0+ WAR in the first half of their respective seasons):

As we can see, the mid-2000s saw a sharp increase in both the overall production of rookies, as well as the number of rookies performing over a certain level. That trend has reached its pinnacle this season, with tons of top prospects being promoted and making good on their potential. There is an issue with just looking at overall WAR across this time period, however: there were fewer teams moving back through the decades, and there could have been different playing time considerations, different handling of prospects, etc. One quite obvious issue is the work stoppages that baseball saw during the late 70s to early 80s, which skews the type of data above.

We can take some of those issues out of the picture by simply using rate stats. With that in mind, let’s first see the share of overall plate appearances rookies accounted for during each season. That will tell us whether that sharp rise in WAR during the mid-2000’s was the result of increased playing time for rookies. Take a look:

Yes, we can see that positional rookies did start getting consistently more plate appearances starting in the mid-2000s. Prior to that, rookies accounting for 10% of league-wide plate appearances during a season was an anomaly; it then became close to the yearly average after 2003. Finally, in the first half of 2015, we’ve seen an incredible spike. The fact is that rookies are seeing a disproportionate share of plate appearances in 2015 compared not only to seasons in the past four decades, but even to the increased rate during just the past decade. By simple visibility, rookies are owning their biggest share of the season out of any in the past four decades.

Now we need to put those two charts together to adjust our WAR for playing time. It makes sense that 2015’s positional rookies have more WAR than any other season, because they’ve accounted for far more playing time than any other. Let’s not simply look at Wins Above Replacement per Plate Appearance (which would be a tiny figure), but go one step further and adjust that figure to assume 600 plate appearances. This is going to include all of the rookies who played during the first half of these seasons, not just the top 20. Here is our chart for WAR/600 for first-half positional rookies:

2015 is good when performance is adjusted for playing time. Really, really good. It’s not quite as good as the incredible back-to-back years of 1986 and 1987, however. 1986 featured first-half rookies Barry Bonds (1.8 first-half WAR), Jose Canseco (3.0), Wally Joyner (3.1), Kevin Mitchell (2.1), and Steve Lombardozzi (1.6). 1987 was no slouch either: Devon White (3.8 first-half WAR), Mark McGwire (3.6), Matt Nokes (2.8), and Kevin Seitzer (2.0) led the pack that year for first-half rookies. Those were exceptional classes, and the names associated with them tell us that 2015 being in the same conversation is really something special.

This season is one we should remember for the sheer joy of seeing so many talented young players come into the league at the same time: given the data we’ve looked at today, it only happens once every few decades. The confluence of a second wild card, many unexpected teams being in the playoff hunt, advanced training regiments, and better scouting have led us to this point. With all likelihood, this season will be one we’ll look back on in a few decades with fondness, remembering the many household names that got their start in the early months of 2015.