Stat This, Not That: How we should be tracking strikeout rates, the most historic numbers of MLB’s current era

The best measure of strikeout prowess isn't actually K/9; a better option — and a record that could be broken in 2023 — is K%

“Give a manager his choice, and he'll pick a pitcher who can strike out batters over one who uses other means to secure outs.”

Murray Chass, the longtime New York Times baseball writer who has attacked the rise of statistical analysis in MLB in recent decades, relayed that kernel of baseball’s oral tradition in the Times in 1983, the year Nolan Ryan and Steve Carlton both surpassed Walter Johnson’s long-standing strikeout record. “Results of pitchers' performances this past season support that choice,” he continued. “Those results show a clear correlation between strikeouts and hits allowed by starting pitchers in both leagues.”

And they did. Chass might not have considered that statistical analysis, but it was. Managers and teams were making decisions based on the way a pitcher secured outs, and he was testing their logic. This is, more or less, the grand story of baseball statistics. Curious observers track everything, then compare word-of-mouth wisdom from newspaper clippings and television broadcasts to quantifiable truths in the records. The most memorable conflagrations in this story focus on moments when conventional thought and the math didn’t line up, but the game’s embrace of — its marriage to — numbers began and persists because the wisdom and the numbers usually align.

One year later, in their seminal book “The Hidden Game of Baseball,” sabermetric pioneers Pete Palmer and John Thorn pointed out that the bogeyman named Stats and the hero known as Knowledge are often the same person in different clothes.

“Even those who profess to abominate statistics — among whom are included several baseball managers, general managers, league officials, ballplayers — are statisticians despite themselves, for we are all, all of us humans, intuitive statisticians.”

Forty years on, there’s no longer a war over statistics’ place in baseball. There’s just the matter of how to best utilize and communicate stats so they don’t overwhelm, so they don’t confuse or antagonize, so they can be valued for what they are: a wondrous, ever-improving tool to appreciate and understand the sport. That’s what this column is going to be about: appraising which numbers are most worthy of your time and attention and explaining what they can add to the game on the field and on the screen.

We’re going to start with strikeouts, a classic measure of pitcher dominance that is having another moment of historic significance right now.

Which strikeout rate is best: K% or K/9?

The all-time strikeout record, as a counting stat, is not in jeopardy. Nolan Ryan played 27 years in the big leagues, many of them in an era when pitchers often threw complete games and eclipsed 280 or 300 innings per season. He racked up 5,714 strikeouts, a number even Randy Johnson fell more than 800 Ks short of, even though Johnson struck out 10.6 batters per nine innings, while Ryan whiffed 9.5 per nine.

Ryan’s extreme longevity, coupled with the league’s “You got it, hoss” approach to arms in earlier generations, led to that outlier number. Some version of that calculus is almost always true with records, even when we don’t want to acknowledge it; individual exceptionalism mixes with a macro condition of the sport.

Right now, the strikeout column in the box score is your best shot at seeing something historic — what home runs were to the steroid era, strikeouts are to our current moment of omnipresent velocity and jaw-dropping breaking balls.

Yet the recent strikeout boom has manifested not in record-breaking totals but in record-breaking rates. Pitchers today simply aren’t trained or allowed to throw 300 innings, but they are better than ever at getting batters to strike out. That is in large part because of what Chass & Co. were realizing in the 1980s: Pitchers who can reliably strike batters out are more reliably good. Many pitchers can limit the damage on balls put in play, but that’s a more fickle game. A strikeout is definitive, the single best thing a pitcher can do. So more pitchers pursue, and are selected for, strikeout prowess.

That prowess is most often measured in the form of strikeouts per nine innings, or K/9. The calculation isn’t hard — multiply a pitcher’s strikeouts by nine, then divide by his innings total — and it provides a pleasingly intuitive result. Seasonal or career averages can mimic the scale of a single-game box score: Your eyes light up at an 11 or 12 in the K column but might simply skip over a humble 7. But the problem with K/9 — besides its waning relevance to the real world, where very few pitchers throw nine innings at a time — is its denominator.

As a statistical measure, K/9 often does give us an accurate look at the best strikeout pitchers, but it isn’t aimed at the target quite as well as it could be. Because it is tied to innings pitched, K/9 really shows what portion of a pitcher’s outs come from strikeouts, not how likely he is to strike out any given batter. As such, a wild or homer-prone pitcher who gives up plenty of baserunners and hard contact but logs all his outs with whiffs could have a higher K/9 than a better pitcher who simply has more ways of winning.

The better alternative, then, looks at a simple percentage of strikeouts against batters faced. That number, strikeout percentage (or K%), correlates more directly to pitcher excellence and is really what teams are interested in maximizing.

To wit: Ryan’s 9.55 K/9 noticeably outstripped Sandy Koufax’s 9.28, but their strikeout percentages were almost identical (25.3% to 25.2%). Excellent for their time, those career K% marks would’ve ranked 15th among qualified MLB starters in 2022 and are just inside the top 20 all time among pitchers with at least 1,000 innings. One step beyond that, K-BB% — the differential between K% and walk percentage (BB%) — is also a useful indicator of dominance that one-ups its more traditional sibling, K/BB ratio.

With that context, you can appreciate that the holy grail for K% in a single season will sound familiar: 40%, or .400. New York Yankees ace Gerrit Cole set the high-water mark for K% in a full, qualified season in 2019, at 39.9%. (Shane Bieber topped it in the shortened 2020 season, but no season of even 100 innings has gotten within a percentage point of Cole’s mark.)

As Aaron Judge showed last season, you don’t have to love the rafts of home runs in the game, on a broad scale, to appreciate an individual pursuit of history. The same theory applies to strikeouts. And while a relatively unheralded rate stat won’t (and shouldn’t) elicit the same wave of enthusiasm as a home run record, there is excitement to be found in a pitcher approaching this pinnacle. And 2023 could present an opportunity to follow just that sort of chase.

There are a couple of clear contenders. First, Jacob deGrom is currently the all-time career leader (minimum 1,000 innings) in both K% (30.9%) and K-BB% (25.1%). For the record, Randy Johnson is the all-time best in K% among retired pitchers, at 28.6%, while Pedro Martinez leads in K-BB%, at 21.0%.

The issue for deGrom has been reaching the 162 innings required to qualify for single-season pitching records. He logged a 45.1 K% in 2021 but threw only 92 innings. A full season from deGrom, in his inaugural Texas Rangers campaign, would be a threat to the K% record.

Likewise, second-year Atlanta Braves starter Spencer Strider astounded MLB last season by striking out 38.3% of batters faced with a devastating fastball-slider combo. He logged 131 2/3 innings after starting the season in the bullpen, but he could conceivably reach record territory this year.

Then there’s Cole. Since reinventing himself with the Houston Astros, Cole has functioned as the high-flying avatar of optimal pitching in the age of optimizing pitching. He gives up a few more home runs than you’d like, but otherwise, he takes every turn, strikes out the world and surrenders very few walks. All of that earned him a $324 million deal in the Bronx, but it has yet to win him the game’s most famous form of non-monetary acclaim; he has no Cy Young Awards.

There’s time, though, for his mastery of pitching’s most important individual skill to sink in — or be proven twice over. Cole’s fastball still averages over 97 mph, and through two starts this season, he has struck out 19 of the 45 batters who stepped to the plate — or 42.2%.

Let the chase begin.

Stat this: Strikeout percentage (K%)

How is it calculated? Strikeouts divided by total batters faced

Where to find it: FanGraphs leaderboards or player cards, on the advanced tab, or Baseball Prospectus leaderboards

Players to watch: Gerrit Cole, Jacob deGrom, Spencer Strider

Weekly stat watch

This might matter: Pablo Lopez, the Minnesota Twins biggest offseason addition (outside of retaining Carlos Correa) and best hope at an ace, is throwing a new sweeping slider on more than a quarter of his pitches. Early returns show it generating a lot of whiffs to go with a fastball with increased velocity. Lopez has averaged 95 mph with his four-seamer in only six of his 96 career starts; five of those have come since September. Velocity changes and confidence in a new pitch are two of the few things we can confidently notice in the season’s first week.

That probably doesn’t: The Phillies have an MLB-worst 6.98 ERA. The defending NL champions got off to a rocky start in Texas, mostly by allowing a bonanza of runs. But the underlying numbers shouldn’t alarm Phillies fans just yet. Aaron Nola and Zack Wheeler looked like themselves; they just had a string of balls in play turn against them.

Now, there might be some legitimate worry about the shape of new bullpen members Craig Kimbrel and Gregory Soto, but that anxiety could go away very quickly this weekend in Cincinnati — or once Ranger Suarez returns to the rotation.

Have a question about stats in baseball? Email or tweet at @zcrizer.

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