On some level, people have understood pitch-framing for a long time. Catchers have long been taught to try to reduce their movement with the pitch on the way, and there were good framers before there was good framing analysis. But before people started really talking about the various good and bad framers, there were some other popular theories regarding the variations in called strike zones. Among them was that home-plate umpires would be tough on rookie pitchers, while to veterans they’d extend the benefit of the doubt. For so long, this was just kind of accepted, even among players, and it’s not like there’s ever been an abundance of data to analyze. That is, until PITCHf/x, which continues to revolutionize things years and years after its launch. We’re all revolutionaries!

Because you can probably see where I’m going, I’ll spare you more needless introduction. In short, I wanted to go in search of the veteran benefit of the doubt, now that we have a full six years of pretty reliable PITCHf/x data. Really, I wanted to go in search of age bias, and I figure if such a bias exists, it shouldn’t be too hard to turn it up. All the information I needed, I got right here on our own leaderboards. And it turns out there’s definitely been something interesting going on. It’s been subtle, but overall it’s been present.

The numbers you’ll see are my own home-spun metric, which I calculate using our own data and which I’ve written about several times before. Here I’m choosing to call it Diff/100, which is the difference between strikes and expected strikes per 100 approximate innings, relative to the league average. With FanGraphs’ PITCHf/x data, it’s possible to calculate how many strikes a pitcher should’ve ended up with. A negative total suggests a smaller called strike zone, while a positive total suggests a bigger called strike zone. According to the theory, we should see inexperienced pitchers get negative totals, while veteran pitchers get positive totals.

I decided to analyze season-to-season by splitting all the pitchers in MLB into groups. For each year from 2008 through 2013, I collected numbers for rookies, guys up to 25, guys between 26-30, guys between 31-35, and guys 36 or older. The groups are somewhat arbitrary, but the arbitrariness shouldn’t mask any meaningful effect. Rookies threw about a fifth of all pitches. Guys between 26-30 threw almost half of all pitches. Guys 36 or older accounted for just 6% of all pitches, but the samples still numbered in the tens of thousands of pitches. At this point I feel like I should just get to the table.

Here’s everything. The numbers are in Diff/100. That’s the number of strikes above or below average relative to the expected number per 100 innings, more or less. Just typing that feels like a mouthful but I promise it’s actually pretty simple.

Group 2008 2009 2010 2011 2012 2013 Average Rookies -1.2 -4.5 -3.5 -7.8 -5.3 -4.5 -4.5 Through 25 -5.3 -2.6 -3.2 -4.0 -3.5 -2.6 -3.5 26-30 -1.0 -1.4 -0.5 -3.0 -0.1 -0.5 -1.1 31-35 7.4 4.8 5.8 3.9 5.5 7.7 5.9 36+ 15 14 14 20 4.8 7.6 13

Right away, you probably looked at the last column, and right away, you probably noticed a bit of a trend. The numbers get more positive as the pitchers get older. The trend basically holds every year, save for 2008, when rookies pitched to a slightly bigger zone than pitchers 25 or younger. The last two years, there’s been no real difference between the second-to-last group and the oldest group, but there has been a difference between those groups and the younger groups.

What the table suggests is that veterans have gotten the benefit of the doubt, compared to younger pitchers. It’s a fairly small effect, considering how many pitches are thrown over an average 100 innings, but it looks like an effect really is there. On average, between rookies and pitchers 36 and older, you’d expect from these numbers an ERA advantage of about 0.20 in the older pitchers’ favor. That is, everything else being equal. That’s roughly the value of 17 extra strikes per 100 frames.

But I have a different, if related theory. My belief is that pitchers with good command will be able to throw to more favorable zones than pitchers with worse command. Catchers will be able to catch them more easily, and umpires will be less often taken by surprise by location. Rookies, probably, will have the worst command, since they’re by and large untested. Then the older groups will be selective for guys with command good enough to stick around, even after their stuff gets worse. Within the oldest group, you have guys pitching in the majors, and you probably don’t have many guys throwing 95 miles per hour. So they have to be good at something else, and it’s probably location. That might well explain what’s observed in the table. It might look like the benefit of the doubt, but really, it’s just easier to get calls if you put the pitch around where its target is.

So there are at least two possible explanations. But there’s only one truth: at least over the past six years, older pitchers, on average, have thrown to more favorable called strike zones than younger pitchers. It’s a relatively small effect, but it’s a tough effect to deny. And regardless of whether it’s because of command or because of umpires giving the benefit of the doubt, it’s still different pitchers throwing to different zones in the same game. A lot of people have a lot of trouble with that, as an acceptable reality. Other people don’t really care, and it’s not like this is going to resolve itself overnight.

Later, I figure it’s worth doing a similar study for hitters instead of for pitchers, since there are also ideas out there about veteran hitters being given a little more leeway. And with hitters, we wouldn’t have to worry about thinking about pitcher command, since you’d expect that would all average out. The results promise to be interesting, or not interesting at all. But they’re going to be published! Unless something terrible happens.