I’m not gonna lie to you — I’d love to be writing about something more compelling. Perhaps some unexplored Clayton Kershaw angle, or perhaps something else entirely. At the end of the day, traffic does have to be there, and traffic follows entertaining, easily-understandable blog posts. It doesn’t so much follow posts about what it’s like, statistically, to pitch to a rookie in Major League Baseball. But we’re all just slaves to the ideas that we have at any given moment, and after I messed around earlier with league-wide leaderboards, I felt the urge to keep messing around in the same place. I promise this is just a phase.
Earlier I did what I could to investigate whether or not veteran pitchers and veteran hitters get the benefit of the doubt from home-plate umpires when it comes to the called strike zone. Those are theories I’ve heard repeated time and time again, and they were easy enough to look into. That got me thinking about other well-worn baseball theories, and I wound up growing curious about how big leaguers have approached rookies, relative to how they’ve approached non-rookies. Do rookies get fed a steady stream of breaking balls? Alternatively, do they get fed a bunch of fastballs in the zone? Is there any meaningful difference in how rookies are pitched to? It didn’t take a lot to put some numbers together.
The FanGraphs leaderboards make things easy because they have a “Rookie” check box which you can toggle or un-toggle. And we’ve got all the necessary league-wide statistics, where, in this case, we’ll be going back to 2002, since that’s where we find our oldest pitch-type data. There’s nowhere to look up the performance of non-rookies, but that’s all easy enough to calculate given rookie performances and given overall performances.
I’ve heard it suggested sometimes that rookies have to prove early on that they can hit slower stuff. I’ve also heard it suggested sometimes that rookies have to prove early on that they can hit hard, challenging stuff. Intuitively, they both can make sense. With regard to the former, rookies usually haven’t seen many big-league-caliber offspeed pitches. With regard to the latter, they also usually haven’t seen many big-league-caliber fastballs, and pitchers like to keep with their fastballs unless they have to throw something else. So a theory is that rookies have to earn the other pitches.
We’ll have to settle for what information we have. Now, two tables. The first spans the majors between 2002 and 2013, excluding pitchers. The second spans the majors between 2008 and 2013, excluding pitchers, covering the PITCHf/x era. Pitch-type data comes from Baseball Info Solutions. Here are the overall pictures:
You’ll notice how close everything is. Everything until you get toward the end of the tables. Rookies have consistently drawn fewer walks, and racked up more strikeouts. They’ve produced at a considerably lower level than non-rookies, with a wRC+ that’s worse by about 15 points. That all makes pretty good sense — rookies have to adjust to the toughest level anywhere, and non-rookies are selective for guys who’ve already had some big-league success. Way to go, non-rookies!
Turn back, now, to the stuff we’re trying to investigate. Over the years, rookies have been thrown more fastballs, very slightly. They’ve been thrown more pitches in the strike zone, very slightly. They’ve been thrown more first-pitch strikes, less slightly but still slightly. It would appear that there’s a small effect, but that rookies have been challenged a little more, relatively speaking.
But, about that last column. Because there are differences in the last column, we should expect there to be differences in some other columns. There’s a fairly strong relationship between wRC+ and fastball rate. There’s a stronger relationship between wRC+ and zone rate, and between wRC+ and first-pitch-strike rate. All of those numbers have at least something to do with the quality of the hitter, and it would appear that rookie hitters are of a lower quality.
It’s hard to establish directionality, here, but just based on the wRC+ difference, we’d expect rookies to have a higher fastball rate by 1.1 percentage points. We’d expect them to have a higher zone rate by 1.0 percentage points, and we’d expect them to have a higher first-pitch-strike rate by 1.1 percentage points. What we observe is that the difference in fastball rate is smaller. The difference in zone rate is smaller. The difference in first-pitch-strike rate is a little bigger.
So that makes it look like rookies have been challenged slightly less often. But what I imagine is going on is that rookies have fallen behind unusually often, ending up in more breaking-ball counts. That is, they’ve fallen behind more than the average 87 wRC+ hitter. Only by a little, but we’re only dealing with a little.
The end result being that it doesn’t look like there’s much of a difference anywhere here outside of the production. Rookies haven’t been fed an unusually steady diet of anything. They’ve been pitched like big-leaguers, big-leaguers who swing below-average bats. Some rookies get more fastballs and some rookies get more breaking balls, but the same goes for non-rookies as well, and it just has to do with one’s perceived batting profile. That becomes known pretty quick, if it isn’t already known when a guy is fresh out of the minors. Teams are constantly scouting, everywhere.
If I had access to better numbers, I’d break this down. A full rookie season lasts a long time, and by the end, the player is hardly a rookie anymore. It would be of particular interest to look at the same data covering just a player’s first, say, month in the major leagues. Toward the beginning is when you’d expect any effect to be at its greatest. As more times passes, you expect things to even out. But if there were a huge initial difference, you’d think that would still show up in the full-year differences, and there’s just not much. Rookies seem to get pitched more or less as they ought to.
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