Front and center: What makes CF special?

Every team in baseball, when setting their defensive lineup in the outfield, has the same idea in mind: Put your best defender in center field. It doesn't always work out that way in practice, due to seniority or clubhouse standing or health, but that's the general idea. The best defenders are in center, and as they age and slow, former center fielders (like Matt Kemp and Andrew McCutchen) get shifted to the corners. No one moves into center except in case of emergency.
It's not complicated to understand why. Center fielders simply impact more plays than the corner outfielders do, and you want your best fielder touching the ball as often as you can. You can see that pretty clearly just by looking at the raw number of total putouts at each position over the past two seasons.
Putouts by outfield position in 2016-17
LF: 17,220 (29.2 percent of outfield putouts)
CF: 22,901 (38.8 percent)
RF: 18,904 (32 percent)
Center fielders make the most plays by a considerable amount. If you take those numbers and look at it annually per team, then the average team's center fielder makes 81 more plays per season than a corner outfielder does. Another way of saying that is that center fielders make 27 percent more plays than their corner-outfield friends.
But we know that and have for years. The question we want to answer today isn't just whether center fielders make more plays, because obviously they do. We want to know why, and we have three theories.
1. Maybe there's simply more opportunities hit to center.

2. Maybe the opportunities hit to center fielders tend to be easier.
   
3. Maybe center fielders are more skilled.
   If it turns out that more than one of those things are true, then how much weight does each "correct" answer get?

It's the perfect question for Statcast™ data. Based on work by MLB.com's Tom Tango, we can look at these through the lens of Catch Probability, which defines the likelihood that a batted ball will be caught based on how far a fielder has to go, how much time he has to get there and what direction -- as well as Outs Above Average, which accumulates all of those individual plays into a season-long aggregated number.
Let's answer those questions one at a time, starting with the easiest.
Are there more opportunities hit to center field?
Yes. By a lot. You can only catch the balls that come your way, and it doesn't matter if you're Byron Buxton or Denard Span in terms of how many of those there are. No outfielder can create his own opportunities.
We don't actually need to spend a tremendous amount of time on this, because the answer is simple. Over the past two years, center fielders have received about 4,000 more catchable balls than right fielders and nearly 5,600 more catchable balls than left fielders. This isn't new, and it's not the interesting part. But it is a good starting point before we move on to more important questions.
Are the opportunities to center field just easier?
If all of the thousands of additional chances that go to center fielders were easy popups or lazy cans of corn, then it wouldn't tell you much about their skill. You could put any outfielder there. You could stand there. One hundred obviously easy chances doesn't make a position more difficult than one that gets a dozen hard chances.
This is something we couldn't do very easily before, because we never had a strong way to know if a play was "difficult" or not. This is where Catch Probability really comes in, because now we know exactly how easy or hard each play was. Aggregate that over the course of a season, and you can know not only how many plays were actually made, but how many plays were expected to be made.
As it turns out … the answer to this one is "not really." Based on the difficulty of all the balls hit to them in 2017, left fielders were expected to make the play 87.6 percent of the time. It was basically identical for right fielders, who were expected to catch 87.5 percent of the balls hit to them, and very nearly so for center fielders, who were expected to catch 88.6 percent of balls. The chances were ever so slightly easier, but by very little.
Where it gets more interesting is when you group the batted balls into three categories, which we'll call "easy," "medium" and "hard."

In the "easy" group, with batted balls that had a Catch Probability of 90 percent or higher, all three positions caught an overwhelming amount of the balls hit their way, over 97 percent. Center fielders performed the best, though somewhat near their expected production.
But as things get harder, center fielders really stand apart. In the "medium" group of batted balls, all three positions were expected to catch approximately 71 percent of balls hit their way, but center fielders caught over 77 percent, well more than the 68 percent of left fielders or the 71 percent of right fielders.
In the "hard" group, it's more of the same. Each position was expected to catch about 17 percent of those very difficult chances. Left fielders (12 percent) and right fielders (16 percent) underperformed. Center fielders overperformed, catching 22 percent. 
Remember when we said that "the average team's center fielder makes 81 more plays per season than a corner outfielder does" above? Based on this additional information, the average center fielder is expected to make 75 more plays per season, and they end up actually making more than that. It would then be fair to say that approximately 92 percent of a center fielder getting to more balls is about the number of and difficulty of chances.

That leaves something else ...
Are center fielders just more skilled than other fielders?
You'd expect the answer to this to be "yes." It is! It's yes. There are exceptions to every rule, but there's a reason that elite defenders like Kevin Kiermaier, Buxton, Ender Inciarte and Jackie Bradley Jr. are playing center, not a corner. When we looked at Sprint Speed by position last summer, center field came up as the fastest position in the game.
An easy way to show this is to simply look at Outs Above Average. Looking directly at Tango's work, we know that over the past two seasons, average center fielders were responsible for plus-6.2 outs above the average outfielder. Corner outfielders, combined, were minus-3.1 outs below average. That's a gap of 9.3 outs between center fielders and corner outfielders, and this a metric that is intended to look at skill.
2016-17 Outs Above Average in MLB
LF: minus-4.8
CF: plus-6.2
RF: minus-1.5
Interestingly enough, you can see a performance split between the corners, too. Left fielders were nearly five outs below average in 2016-17, while right fielders were only slightly below average. Intuitively, this makes sense; strong non-center fielders like Mookie Betts and Jason Heyward often move to right, not left.
So to recap: We've learned that center fielders make considerably more plays than their corner-outfield mates. It's somewhat due to skill and overwhelmingly due to opportunities. It's not really due to difficulty of opportunities that much, but on the plays that are difficult, that's where center fielders truly set themselves apart with speed and skill.
As with everything else, there's no one-size-fits-all rule here. Some teams don't have their best fielders in center, due to roster or personal concerns. Some players are more comfortable in one spot or the other. But the age-old simple idea of "put your best outfielder in center" still holds up. After all, you want your best defender getting all of those additional touches.