There’s a fundamental truth about the playoffs that everyone knows, but that’s also easy to forget. That is, the playoffs are selective for the best. They’re selective for the best baseball teams, and as a part of that, they’re selective for the best baseball throwers and the best baseball hitters. Come playoff time, it doesn’t matter so much that, say, the Cubs trounced all their opponents, because in the playoffs, everyone is at least pretty good. The quality of competition evens things out. Lineups are disproportionately good, but so are pitching staffs, and it can feel almost like a whole month of partially-diluted All-Star Games.

October selects for the best on-field product. But what about the on-field umpires? We know that umpires have to earn their way to gaining postseason responsibilities. We know it’s considered an honor to be an ump in the playoffs, as it should be. You’d think it would follow that the playoff strike zone would reflect this selection on the part of the league. As I look at it, the October zone does appear slightly different. But not in the direction of being better. Rather, it seems a little more pitcher-friendly.

As I always try to make clear in posts like this: I’m not actually a good analyst. There are some really good strike-zone analysts out there, and I’m not one of them. I look for easy, simple solutions, and pray they’re not misleading. That’s what I’ve done here, using Baseball Savant. If you’ve ever used that tool, you know it allows you to select the designated PITCHf/x strike zone. I accepted that as my foundation, and I proceeded. Let’s just get right to it! I looked at data from 2008 through 2016, separating the regular season and the playoffs. First, here are the rates of in-zone strikes, over all in-zone called pitches.

On its own, that looks good, right? This is what we would want to see. The playoff line is consistently a little above the regular-season line, indicating that fewer deserved strikes have been missed in the playoffs. Over the window, the regular-season average is 88.0%. The postseason average is 89.2%. It’s small, but it’s meaningful, and so come playoff time, strikes are more often strikes. Good job, umpires! Now for the rates of out-of-zone strikes:

The trend is good — as you could see in the first plot, umpires have been getting better over time. But this doesn’t quite show what you’d want to see, necessarily. If there are more in-zone strikes, you’d also want to see fewer out-of-zone strikes. Yet here, the line for the postseason is again pretty consistently a little above the line for the regular season. Since 2008, the regular-season average is 14.6%. The postseason average is 15.1%. That’s a smaller gap than before, but, there are way more pitches taken out of the zone than within it. So why not put this information together? Here are the year-to-year “correct-call” rates, assuming, again, that we can trust the strike-zone coordinates:

The umpires have gotten better about the zone, as it’s been designated. What we actually know this to mean is that the strike zone has gotten considerably bigger during the PITCHf/x era, but that’s kind of because the strike zone itself is fairly large. That’s why there’s been talk about shrinking it. By the data on Baseball Savant, in 2008, 83.8% of calls in the regular season were correct. This past year, that was up at 87.6%. There’s been very steady improvement in that regard.

But look at the blue line against the green line. There’s effectively no separation. The regular-season average is 86.0%. The postseason average is 85.9%, yielding a negligible difference. Just this year, we’re looking at 87.6% against 87.4%. The strike zone in the playoffs hasn’t been meaningfully more correct. Though calls within the zone itself have been more accurate, those have been offset by a slight widening of the zone boundaries. Put another way, pitchers have found strikes in more places, and that’s presumably worked a little to their collective benefit.

I’m going to guess this isn’t actually too hard to explain. For one thing, playoff umpires aren’t only selected for their strike-zone accuracy. Not in the way that players are selected for their performance. But beyond that, we know that playoff pitchers are better, overall, and better pitchers tend to have better command. Pitchers with better command are far easier to receive, and pitchers who are easier to receive will get more of the benefit of the doubt. It’s possible, also, that the playoffs somewhat select for better receivers in the first place, but I think a lot of it is on the arms. In the playoffs, you see fewer Trevor Bauers, and more Corey Klubers. You still see some Bauers, but not nearly so often, and predictable pitchers are easier pitchers for catchers and umpires alike. A pitch just off the plate by Bauer probably looks a little less like a strike than a pitch just off the plate by Kyle Hendricks.

That doesn’t make anything *good*. It just is, assuming that’s an accurate explanation. And I’m not sure there’s much to be done about it, so long as baseball continues with its current system. Based on this preliminary analysis, the typical postseason strike zone favors the pitcher more than the regular-season equivalent. It’s so subtle you might never notice from home, but, add it onto the pile of factors that make hitting in the playoffs so incredibly hard.