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Baseball is a game of countless axioms. You can never have enough pitching, you shouldn't make the first or third out at third base, and good things will happen if you throw strikes. Much of this wisdom has been challenged over the years, some of it verified. One self-evident truth is tested more in September, with its expanded rosters and flabby lineup cards, than in any other month: pinch-hitting and starting aren't the same. Considering their rarity—particularly in the American League—it's impressive that any truism about pinch-hitters could tally considerable mileage, yet one does. To wit:

You’re not really afforded to look for pitches as a pinch-hitter. I was actually kind of mad at myself for not swinging at the first-pitch fastball." – Tucker Barnhart

"If you get a first-pitch fastball you can hit hard, you’re not waiting around. You’d like to be able to see some pitches and get a little comfortable, but you can’t afford to do it.” – Chad Tracy

"[Hitting coach Pedro Grifol] has been doing a good job of preparing these young guys about pinch-hitting. First fastball in the strike zone, you better be ready to hit, and David was last night. First-pitch fastball—boom, base hit. That's what a good pinch-hitter does." – Ned Yost

"Hector Lopez used to say, 'swing at the first fastball you see. You'd better go up there looking for a fastball.'" – Archie Moore

Rookies, veterans with more than 200 pinch-hit at-bats to their account, managers, and players from past eras sing in agreement: Pinch-hitters must take advantage of the first fastball. The thought process makes sense—after all, the oversimplified key to a good approach is getting a hittable fastball—and sounds good when other factors are considered. However, the axiom itself is not grounded in reality, at least not these days: according to Andrew Koo's research, pinch-hitters have swung at one more first fastball per 100 pitches than starters over the past three seasons. But while the pinch-hitter choir gets the words wrong, their message—pinch-hitters stray from the norm—is correct.

Most of the statistical differences between starters and pinch-hitters border on imperceptible. For instance, pinch-hitters swing at 46 percent of the pitches they see—the same as starters—while swinging at 64 percent of the pitches they see in the zone—an additional swing per 100 pitches compared to starters. The gap between the sides only becomes noticeable when putting the bat on the ball is examined. That's because pinch-hitters' contact rates are five percentage points worse than when those same hitters are starting.

Why is the difference between contact rates greater than everything else? If pinch-hitters were more likely to swing, or leastways more likely to expand the zone, then the contact rate slippage would be self-explanatory. On a macro basis, though, hitters seem to maintain the same tendencies. So what gives? There seem to be three contributing factors:

1) The pitchers. Duh. Most pinch-hitters tend to face a certain kind of pitcher—i.e., relievers—which means less contact is expected before they step to the plate. Or, as Chad Tracy tells it: “These guys are one-inning guys. Their breaking balls are sharper. Their heaters are harder. They’re coming in trying to strike you out or embarrass you." There's also the platoon advantage, especially in the NL, where pinch-hit decisions are telegraphed by the pitcher's slot coming up in a late-and-close situation.

2) The situation. Although the overall tendencies might remain the same, it's possible the hitter's pitch selection changes in ways unaccounted for. Hypothetically speaking, a batter might be more willing to swing at a first-pitch curve, or a fastball down-and-away if a hit is deemed even more valuable than a walk than usual. Likewise, batters would be more likely to swing from their heels—thus increasing the likelihood of a whiff—if a home run could tie or win the game. This explanation loops back to what Russell Carleton wrote about last month, on how clutch hitting might exist, albeit in a different manifestation.

3) The other aspects of the pinch-hitter penalty. Facing live pitching after a few hours spent on the bench isn't easy. It seems reasonable to think a hitter's focus (and bat control) might not be as sharp as usual. Besides, many pinch-hitters are worse hitters than your starters anyway.

Weigh those factors to your own liking, but altogether they would seem to offer a decent explanation for why, with almost all else equal, pinch-hitters tend to swing and miss more than starters. Hence why batters have been found to lose 34 points of wOBA when used as a pinch-hitter, according to The Book.

So the next time you're watching a game and a pinch-hitter is summoned, don't expect to see him act much different than he would otherwise. Instead, prepare for a little less contact and much more of the same.

Special thanks to Andrew Koo for research assistance

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How do the differences stack up to a starters first AB only? Subsequent times through the lineup sees increased wOBA. And the first AB is most similar to a pinch hit appearance in that you don't have the mental image of the pitchers stuff yet. Just a thought.
Interesting how little some things change, just as your comments about "players from past eras" suggest. I remember, a long time ago, reading an interview with the great pinch hitter Smoky Burgess, while he was still active. The title was something like "Burgess on pinch hitting: Swing at the first pitch" -- don't quote me on that, but I do remember the next line: "Pitchers will usually lay it in there." This was just about fifty years ago...
This is one (of many) examples how people's recollection or characterization of what they say or do is simply not reliable.

I mean if what pinch hitters say they do is simply not true (which it is not), why would we believe anything that a ballplayer or ex-ballplayer says about what they did or thought?

That is one reason why you hear so many stupid things coming out of the mouths of these ex-player commentators on TV and the radio (and internet), even when it specifically relates to their profession (thus you would think that they were reliable sources of information).

People who actually do, say and think the things that are contained in narratives are just as likely to believe those narratives as are people who have nothing to do with them.

There are SO many things that ballplayers will tell you they do or think that simply aren't true, that we have to remember that people's recollections, even from the so-called experts, are not substitutes for actual data. In fact, they are often the least reliable sources of information.

"In God we trust. Al others must bring data." William Deming.