The Continuing Rise in Strikeout Rate

In yesterday’s post on the early home run trend of 2013, I noted that strikeout rate was up again, as it has been for a while. At this point, the continuing rise of strikeout rate isn’t a new story, and I think most of you are probably aware of the fact that Major League Baseball is essentially setting a new record high for league average K% each season.

One of the main theories that is espoused for why strikeout rate is ever increasing is the simultaneous increase in pitcher velocity. It used to be that Randy Johnson was a freak because he could touch 100 mph with his fastball, but now it seems like every team in baseball has a guy who can hit that mark. While we don’t have historical velocity data, we do have PITCHf/x velocity data since 2007, and BIS velocity data going back to 2002. While there are some differences due to the classifications of pitch types, both support the idea of rising velocity.

The BIS data has the average fastball going from 89.9 in 2002 to a peak of 91.6 last year, a nearly 2 mph rise in the average fastball speed over the last 11 years. We know that velocity and strikeout rate are highly correlated, so a league wide rise in the speed of pitches would explain why strikeout rate keeps going up and up.

However, we also know that average velocity generally starts lower in the coldest month of April, and tends to rise as the season goes on and the summer gets warmer. So, since we were already digging into the trends of monthly data versus full season, I thought it might be worth looking at whether strikeout rate follows a similar path. If velocity is driving the rise in strikeout rate, and velocity is at its lowest point in April, then it should follow that strikeout rate should also be at its lowest point in April.

To test out whether that’s true or not, I got our pal Jeff “Not The Pitcher, Player Linker” Zimmerman to send me monthly strikeout data back back to the start of the 2002 season. Here are the average K% totals for each month of the season from 2002 to 2012:

Month K%
April 17.5%
May 17.2%
June 17.3%
July 17.5%
August 17.6%
September 18.3%
Overall 17.6%

During the first five months of the season, there’s no real K% trend. K% in April has actually been slightly higher than it has been in May or June, and then it has gotten back to April rates by the middle of the summer. There is, however, a huge spike in September. The most obvious explanation for the September strikeout spike is expansion of the rosters, as the addition of more players allows managers to play more match-ups in selecting relievers, and the addition of additional minor league bats might be driving up the strikeout rate for hitters as well. But, putting that structural change aside, it doesn’t appear that there’s a significant difference in strikeout rate between the beginning of the season — when average velocity is at its lowest — and later in the summer, when pitchers are throwing harder.

Perhaps a chart will be beneficial here. Here is the overall rise in strikeout rate from 2002 to 2013, by month:


The September spikes stand out, but there’s another thing that chart shows that I hope you’ve noticed — the drastic increase in K% since 2008. From 2002 to 2007, K% hovered right below 17% and was trending up at a slow pace, but then it took off in 2008, jumping to 17.5%, then 18.0%, then 18.5%, and so on and so forth.

Now, here we stand in April of 2013, and despite an average fastball velocity that is equal to what the league posted overall in 2010, we’re on track to have the second highest K% month in the sport’s history, second only to last September. So, what happened in 2008 that continues on to this day, and doesn’t seem to be driven primarily by the rise of velocity throughout the summer?

Well, 2008 was the year after MLB Advanced Media and Sportvision installed PITCHF/x cameras in Major League Stadiums, and it was the first year that umpires could potentially have been acting upon a directive from the league based on that data. MLB had previously been using the Questec system to evaluate umpires, but the system was only installed in about a third of MLB ballparks, according to this 2009 article from the New York Times. In that article, former head of umpires Mike Port described how the “Zone Evaluation” system would be an improvement:

“It’s an upgrade from where we were,” Port said in a telephone interview. “The umpires, they don’t want to miss a pitch any more than a batter wants to strike out. Where the Z.E. system will give us a lot of help is more data to help identify any trends: ‘The last three plate jobs, you missed seven pitches that were down and in. Here’s how one of the supervisors can help you adjust your head angle or your stance to have a better chance of getting those pitches.’”

Over the last 30 years, the strikeout rate in MLB has gone from 14.0% to the 20.0% it stands at today. It took 24 years to move from 14% to 17%, but it’s only taken six years to move from 17% to 20%. Those six years correspond perfectly to the PITCHF/x era.

And, while correlation is clearly not the same as causation, there is other data that suggests that the rising strikeout rate could very well be attributable to changes in umpiring. James Gentile authored a piece on Beyond the Box Score a few weeks back that showed that called strikes are increasing at a much higher rate than swinging strikes. Borrowing an important image from that piece:


Swinging strikes are up too, but called strikes have risen at a higher rate, and of course the two things would be somewhat linked; if there are more called strikes, there are more pitcher’s counts, and batters are more likely to chase pitches out of the zone than they would have had the called pitches gone the other way.

As Gentile notes, the fact that called strikes are up does not necessarily mean that that umpires are simply calling a larger strike zone, as pitchers could be trusting their new faster velocities more and nibbling less, or perhaps they’re just getting better at hitting their spots. This data doesn’t prove that the strike zone is expanding.

But, again, we have PITCHF/x data since 2007, and if pitchers were pounding the zone more often in order to cause the rate of increase in called strikes, we should be able to find it in the data. Since 2007, here’s the rate of pitches that have been labeled as in the strike zone by PITCHF/x:

Season PA Zone%
2007 188623 49.3%
2008 187631 49.9%
2009 187079 50.1%
2010 185553 50.1%
2011 185245 49.8%
2012 184179 49.2%
2013 9828 48.4%

PITCHF/x isn’t perfect, and there is a margin of error on these numbers, but I don’t see a significant rise in pitches in the strike zone there. Again, this isn’t conclusive, as the called strikes could be happening earlier in counts, leading pitchers to bury more pitches out of the strike zone later in counts, with those effects offsetting to hide themselves within the aggregate data. This just isn’t a granular enough study to conclude that the strike zone has gotten bigger.

But, I think the data is at least pointing in that direction. The drastic uptick in strikeouts since PITCHF/x cameras were installed seems like a pretty big coincidence, especially since the called strike is on the rise more than the swinging strike. If the main driver of the increase in strikeout rate was velocity or simply more hard-to-hit pitches, it seems like we’d see a trend in K% as velocity climbed throughout the season. If hitters were more willing to trade strikeouts for a better chance at a home run, it seems like that trade-off would be more pronounced in the summer, when the ball actually travels better and rewards that philosophy more often.

If the main driver of strikeout rate is not the players, though, then we wouldn’t see the environmental effects on temperature and velocity in strikeout rate throughout the season. And, looking at the monthly data, we don’t really see those effects. That in itself isn’t enough to prove that the guys driving strikeout rate up are the umpires and not the players, but the timing of the strikeout leap seems to suggest to me that PITCHF/x data is having a larger impact on the game than perhaps anyone anticipated.

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Dave is a co-founder of and contributes to the Wall Street Journal.

39 Responses to “The Continuing Rise in Strikeout Rate”

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  1. LionoftheSenate says:

    Nice work.

    Remember when tons of people were foolishly crying about Questec being in ball parks? Typical, lots of people rip baseball for every move it makes. The people that ripped Questec look as silly as ever.

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    • eye-roll says:

      One of those who protested most spectacularly was Curt Schilling; yet this suggests he should have been more helped than hurt by it overall.

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    • Bill says:

      Schilling’s complaints were that umpires called games differently when Questec’ed than they did without. He was upset at the lack of consistency. With well calibrated machines in every park, this is no longer an issue. If you read the SI article linked in eye-roll’s post, you see that the Questec system was flawed. It sounds like it relied on a human operator. The people that ripped Questec weren’t largely ripping the idea, they were ripping the implementation. They helped to bring about the change to the far superior Pitch-fx systems now installed. Pitch-fx doesn’t make the Questec rippers look silly, it makes them look right.

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  2. DD says:

    This is great, but I don’t think you can look only at K rates for the answer. Are BB rates also dropping? Are most key offensive numbers down as well? This may indicate that the level of hitters has cycled to a recession (or pitcher quality has risen).

    Also: “Again, this isn’t conclusive, as the called strikes could be happening earlier in counts, leading pitchers to bury more pitches out of the strike zone later in counts, with those effects offsetting to hide themselves within the aggregate data.”

    Can we see the First Pitch Strike % to address this? Maybe % of 2-strike counts?

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    • Dave Cameron says:

      Walk rate is basically unchanged. As we talked about yesterday, power was down a bit in the immediate aftermath of the implementation of PED testing but has trended back up lately. Besides the dramatic change in K%, there’s not a lot of difference between offensive numbers now and 10 years ago.

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      • JP says:

        I’m not sure what you mean by ‘Besides the dramatic change in K%, there’s not a lot of difference between offensive numbers now and 10 years ago.’ BB are down over 9.5% since 2002 and runs are down 6.5%. I think strikeouts are up over 15%, but to say that the downward trend in walks is not a lot of difference seems like you are not taking into account how much the drop in walks is contributing to the lower scoring environment we are now in.

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  3. olethros says:

    Clearly, the only reasonable conclusion to draw from this data is that all hitting records from pre-2008 are invalid, as they were achieved with the aid of performance-enhancing umpires.

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  4. mattsd says:

    How has swing % changed over this same period?

    Are hitters taking more pitches, which could explain in part the rise in called strike %?

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    • Huisj says:

      I had the same thought. The shift toward thinking in terms of getting on base and hitting for power could have changed approaches at the plate where people are patient but also swing hard. While more called strikes can lead to more pitchers counts, could a drop in swing percentage also lead to more called balls too? Are pitches per plate appearance up at all? It seems that increased patience combined with harder swings could logically lead to more strikeouts while walks would hold steady.

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      • Paul says:

        Walks have not held steady. Dave is downplaying the drop in BB%. Swing rates are not significantly different since 2007. The only real change in hitter approach is that contact rate was down significantly last year due almost entirely to a drop-off in outside zone contact.

        So no, there is no evidence whatsoever that sabremetrics has caused MLB hitters to adopt a beer league approach, while there is obvious and overwhelming evidence that Pitch F/X has caused umpires to call a more true strike zone. I don’t expect the narrative to change, though.

        To Dave’s question about pitcher velo. Doesn’t it make sense that if the rule book strike zone is being called, pitchers are not going to baby pitches on the corners and especially in the upper part of the zone?

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  5. Robert Norton says:

    I think a glaring hole in this is scouting done by pitchers before a game based on the huge amounts of data that have been piling up over the years on what pitches strike batters out.

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  6. Greglpdx says:

    Wouldn’t the introduction of PITCHF/x also mean that the pitching staffs would have more information on how to attack the batters? Whether it’s more detailed information on the opposing batters or simply more feedback on a pitcher’s mechanics, wouldn’t that be a significant factor? I might be reading the K% chart wrong, but it looks like April 2008 was still relatively low, while the increase starts in May. That might make sense since there would be very little PITCHF/x data at first.

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    • blwfish says:

      Wouldn’t Pitch/FX also provide hitting coaches with just as much new information for the hitters to adjust to pitchers?

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  7. Cidron says:

    I wonder if the crackdown (at least publicly) on PED’s in the 2000’s might have some affect as well. Yes, PitchF/x went in, but PED’s went out at a similar time…..

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  8. Baltar says:

    What do you think of Bill James’ theory on increasing K’s? Poorly summarized by me: Teams are looking for high strikeout pitchers because strikeouts are a good indicator of pitchers’ value, but not looking for low strikeout batters because strikeouts are not a good indicator of batters’ value.

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  9. Ruki Motomiya says:

    Two theories that may or may not be logical:

    1. Hitters are more willing to take a pitch and work the walk now, which means looking at more pitches, ergo creating more called strike from when they look at it and think it is either a ball or it ends up a strike and they don’t have time to swing. Results in a similiar BB%, higher K%.

    2. The one I personally like: Hitters are more willing to just strikeout now, because a strikeout can be better or equal to a normal strike in a fair amount of instances (Man on 3rd is the one I can think of where hitting for an out is pretty much always better than a K). Can’t hit into a DP with a K, after all. And even if it isn’t really ever better, it seems like there would not be a big difference between outs made by Ks and outs made by hitting the ball, for the hitter anyway.

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    • LoydKristmis says:

      I like #2 here, maybe with coaching possibly having an impact. Since saber friendly ideas have finished being adopted by all teams in the last decade to varying degrees, are more coaches are encouraging violent swings?. Let’s call it the Zobrist effect. It could tie together Dave’s HR article from yesterday with this one involving K’s. The correlation between higher K% and higher offensive production is well established. Put simply, it is worth it to swing harder. Combine that with the higher velocity and PitchFX and a pretty solid picture is forming.

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  10. TKDC says:

    I can’t tell if this chart indicates that there is more of an upward trend on the month-to-month level during the past 5-6 years (as opposed to just year-to-year). Is that true, and if so, could it mean anything?

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  11. Anthony says:

    Looking at the pitch type league stats from 2008-2012 there has been a decrease in four seam fastball percentage every year,from 60.7% in 2008 to 57.6% last season. With the assumption that the league’s umpires have improved, this would support the notion that pitchers have more confidence that breaking pitches will be called strikes.
    Since the pitch value as calculated here has a negative value for four-seam fastballs the pitchers could have adapted to use more breaking pitches, as they have a better average outcome, especially when they are called as strikes.
    Although I cannot find any information here on batter swing selection here, i.e. percent of fastballs swung at vs percent of sliders or curveballs, if there was a significant difference in swing selection, which I would assume there is, that could go towards explaining the increase in called strikes.
    Since pitchers are in charge of their pitch selection, which has changed consistently over the past few years, it would make sense that there has been a change in the strikeout rate by way of called strikes, and the consistency in swinging strikes could be evident that hitters have not changed their approach as much as pitchers have, although that can’t be proven with this data.

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  12. Hurtlockertwo says:

    I blame it on Ritchie Allen. Besides his surly personality, I think Ritchie (Dick) Allen never got his due because he was one of the first real stars who struck out a lot. (he w as pre-Bobby Bonds, pre-Reggie Jackson)

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  13. Jay29 says:

    Dave, since we know K% has been increasing, it stands to reason that it might not be done increasing. What, if anything, do you suppose is an upper limit to league K%? Would the league step in and change something (a la 1968) if it got above a certain threshold?

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    • Nathaniel Dawson says:

      Good question. You would think that teams would self-regulate that by player selection. A player that strikes out too much and isn’t compensating for it by walking and/or hitting for power often enough probably isn’t going to play much. Strikeouts can’t continue to rise indefinitely; as strikeouts rise to the point of hurting offense, teams will start to shift toward players that are better at making contact.

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  14. rtcrules says:

    There were 2 possible explanations I thought of. First, teams hold their minor leaguers down for arbitration reasons and hold onto the guys who are out of options so they may not have the best 25 man roster available out of the gate. Secondly, I have always, and possibly incorrectly believed that it takes starters a few starts into the season to build up their stamina allowing relievers to throw more innings. Relievers have higher strikeout rates pushing the early season numbers up.

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  15. Matt Gray says:

    After reading, I have a few thoughts :
    What is the trend of pitchers being drafted? It seems like teams have been drafting more pitchers, and pushing elite talent towards pitching (C. Kelley)

    Increase use of relievers, due to decreasing pitch counts for starters. The “golden” standard for Starters is 200 innings, 20 years ago that was simply expected. Teams have learned you can win a game by closing the door from the back end. More relievers, more fresh arms, higher velocity output during games. (Orioles, Braves, Red Sox etc.)

    We should consider pitch selection, baseball goes in cycles. We saw the split finger era, slider era, cutter era, 2 seam era… Pitchers not longer go for the strikeout as much, their has been a philosophical change around the league for efficiency. (more groundball pitchers)

    Teams are know more accepting of strikeouts because of the need for power, the return of the classic slugger (old man skills, M. Reynolds)

    Finally, I believe some of the data is skewed, from 1994-2008ish steroids/PEDs ruined the game. Yes, pitchers used ‘roids too, but pitching is a unnatural violent motion versus hitting which is less destructive to the body. Pitchers were more likely to incur an injury. Fewer pitchers had enough gas to beat the hitter… Higher pitcher velocity, quicker & stronger hitters, equals the ball going further off the bat. (# of 50 hrs season from ’94 to present)

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  16. CircleChange11 says:

    It could also be that more batters are resisting swinging at pitches they cannot hit well. Gone are the days where “putting it in play” is the end-all.

    Velocity has to be a major factor.

    I would also suggest that the decline in PED use has caused pitchers to be more aggressive, but I don;t know that I can really prove that.

    I would also guess that slider usage has increased and is IMO the most difficult pitch to hit.

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  17. aaaa says:

    How about focusing on the movement of the ball? Balls with more movement may tend to make batters hesitate to swing.

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  18. TKDC says:

    Another factor at play could be better defense and defensive positioning, which makes it harder to have value as a gap hitter or high contact guy alone. You can’t defend against a home run, so the more teams are able to defend against non-home runs, the more value there will be in home run hitters. Of course, I don’t think BABIP has changed much league wide. Still, though, you would expect guys who hit the ball hard to have better BABIP, so teams could still be getting better at preventing balls in play from dropping while at the same time giving up just as many hits on balls in play.

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  19. JS says:

    Sorry if I missed this (either here or somewhere else on the site): Is there any evidence that umpires have actually improved since the adoption of Pitch f/x? Are they making fewer errors?

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