Summary: Based on a suggestion heard at SaberSeminar, I use a few different means to examine how rookie umpires call the strike zone. Those seven umpires appear to consistently call more low strikes than the league as a whole, but some simple statistics suggest it’s unlikely they are actually moving the needle.
Red Sox manager John Farrell was one of the speakers at Saberseminar, which I attended last weekend. As I mentioned in my recap, he was asked about the reasons offense is down a hair this year (4.10 runs per team per game as I type this, down from 4.20 through this date (4.17 overall) in 2013). He mentioned a few things, but one that struck me was his suggestion that rookie umpires calling a larger “AAA strike zone” might have something to do with it.
Of course, that’s something we can examine using some empirical evidence. Using this Hardball Talk article as a guide, I identified the seven new umpires this year. (Note that they are new to being full-fledged umps, but had worked a number of games as substitutes over the last several years.) I then pulled umpire strike zone maps from the highly useful Baseball Heat Maps, which I’ve put below. Each map shows the comparison between the umpire* and league average, with yellow marking areas more likely to be called strikes and blue areas less likely to be called strikes by the umpire.
* I used the site’s settings to add in 20 pitches of regression toward the mean, meaning that the values displayed in the charts are suppressed a bit.
The common thread, to me, is that almost all of them call more pitches for strikes at the bottom of the zone, and most of them take away outside strikes for some batters. Unfortunately, these maps don’t adjust for the number of pitches thrown in each area, so it’s hard to get aggregate figures for how many strikes below or above average the umpires are generating. The two charts below, from Baseball Savant, are a little more informative; red dots are the bars corresponding to rookie umps. (Labeling was done by hand in MS Paint, so there may be some error involved.)
The picture is now a bit murkier; just based on visual inspection, it looks like rookie umps call a few strikes more than average on pitches outside the zone, and maybe call a few extra balls on pitches in the zone, so we’d read that as nearly a wash, but maybe a bit on the strike side.
So, we’ve now looked at their strike zones adjusted for league average but not the number of pitches thrown and their strike zones adjusted for the relative frequencies of pitches but not seriously adjusted for league average. One more comparison, since I wasn’t able to find a net strikes leaderboard, is to use aggregate ball/strike data, which has accurate numbers but is unadjusted for a bunch of other stuff. Taking that information from Baseball Prospectus and subtracting balls in play from their strikes numbers, I find that rookie umps have witnessed in total about 20 strikes more than league average would suggest, though that’s not accounting for swinging vs. called or the location that pitches were thrown. (Those are substantial things to consider, and I wouldn’t necessarily expect them to even out in 30 or so games.)
At 0.12 runs per strike (a figure quoted by Baseball Info Solutions at the conference) that’s about 2.4 runs, which is about 0.4% of the gap between this year’s scoring and last year’s. (For what it’s worth, BIS showed the umpires who’d suppressed the most offense with their strike zones, and if I remember correctly, taking the max value and applying it to each rookie would be 50–60 total runs, which is still way less than the total change in offense.)
A different way of thinking about it is that the rookie umps have worked 155 games, so they’ve given up an extra strike every 8 or so games, or every 16 or so team-games. If the change in offense is 0.07 runs per team-game, that’s about one strike per game. So these calculations, heavily unadjusted, suggest that rookie umpires are unlikely to account for much of the decrease in scoring.
So, we have three different imperfect calculations, plus a hearsay back of the envelope plausibility analysis using BIS’s estimates, that each point to a very small effect from rookie umps. Moreover, rookie umps have worked 8.3% of all games and 8.7% of Red Sox games, so it seems like an odd thing for Farrell to pick up on. It’s possible that a more thorough analysis would reveal something big, but based on the data easily available I don’t think it’s true that rookie umpires are affecting offense with their strike zones.