RESEARCH: Why the leaguewide strikeout rate will finally reverse trend in 2015

It is no secret that offense has declined and strikeouts have risen dramatically across Major League Baseball in recent years:

The mystery that many analysts have been trying to solve is why. Attempted explanations include the instillation of performance enhancing drug testing, wider adoption of defensive shifts, quicker calls to the bullpen for harder-throwing relievers, increased specialization resulting in more pitcher-advantaged platoon matchups, changes to how umpires call balls and strikes due to PITCHf/x, heavier usage of the cut fastball, plate adjustments by batters leading to more swings-and-misses and more first pitch strikes, and other more speculative root causes. All have some grounding in truth. But none have been able to fully explain the reduction in runs or the rapid rise in strikeout rate, nor have any analysts (as far as I know) been willing to issue a firm prediction about how long either will continue. Until now.

I believe that we are currently experiencing "peak strikeout" in the major leagues. And short of some unexpected new systemic change (e.g., higher mound, automated ball/strike calls, wood bats replaced with foam noodles), then it seems highly unlikely that the year-over-year rise in strikeouts will continue in 2015. Here's why:

Over the last 15 years, there has been a very clear difference in the strikeout rate of incoming players (active this year but not last year) vs. outgoing players (active this year but not next year) vs. established players (active last year, this year, and next year) vs. single-season players (active one year but not the year before or after):

Established batters have the lowest K% of any cohort. Incoming and outgoing batters have roughly the same K% overall, while single-season batters have by far the highest K%. These findings are intuitive and in line with our understanding of batter aging with respect to contact rate (i.e., both younger and older players have higher strikeout rates than peak age players).

On the pitching side, established pitchers have the highest K% of any cohort, but incoming pitchers have caught up to them in recent years. Outgoing pitchers and single-season pitchers have much lower strikeout rates, though K% among outgoing pitchers has risen steadily over the last 15 years.

While these high-level differences are interesting in and of themselves, they don't tell the full story. What we really need to know is the true difference in strikeout "churn" between incoming and outgoing players, and the degree to which any differences "stick" in subsequent years as incoming players join the "established" cohort.

To this end, I used Retrosheet's event files to compare the performance of incoming batters to that of last year's outgoing batters, and similarly to compare the performance of incoming pitchers to that of last year's outgoing pitchers. To make the comparisons as fair as possible, I measured how these cohorts performed vs. established players and how they performed directly vs. each other. Here are the results for these batter-pitcher pairs (E=Established, I=Incoming, O=Outgoing, where the batter designation is the first initial in each pair):

  • Incoming batters consistently strike out more often against established pitchers than outgoing batters did the previous year.
  • Incoming pitchers consistently strike out far more established batters than outgoing pitchers did the previous year.
  • Plate appearances featuring incoming batters vs. incoming pitchers end in strikeouts more often than plate appearances featuring outgoing batters vs. outgoing pitchers did the previous year.

Combine this K/PA churn with the simple fact that an increasing percentage of total MLB plate appearances have been coming from "incoming" players (batters and/or pitchers) ...

... and we can roughly model the expected leaguewide K/PA due to the annual churn of players, assuming that these differences accumulate over time as one year's "incoming" group joins the "established" group in the years that follow. (While this seems like a big assumption, note that single-year players with higher strikeout rates have been deliberately excluded from the churn calculations, and that the next year's calculations do include the previous year's incoming players among the established players. The effect of aging on K/PA among established players has not been explicitly factored in; given that contact rate rises through a batter's late 20s before beginning its slow decline, and that K/PA among pitchers declines steadily year-over-year, the net effect would likely be a small-to-moderate tapering of the cumulative effect.) The result (using actual 1999 strikeout rate as a starting point for the expected K/PA trend) is illuminating:

Now, it is easy to look at these charts and think "Okay, this increase due to churn clearly happens every year, so it will likely just keep happening as higher-strikeout new players keep replacing the lower-strikeout older players." There's just one problem, though:

We're fresh out of new players.

More accurately, we're out of new players who can reasonably be expected to continue to drive the major league strikeout rate further into the whiffosphere. Here are the leaguewide strikeout rates of every level of the minor leagues for the same time period that we've observed the rise in major league strikeout rate:

Look familiar? The rise in leaguewide strikeout rate extends all the way down to the lowest levels of the minors. When expressed as an index to the major league strikeout rate, though, the problem becomes crystal clear:

Levels of the minors that had produced strikeout rates 10-20% higher than those of the major leagues now equal or—in the case of Triple-A, Double-A, and Advanced-A levels -- have even fallen BELOW the major league strikeout rate over the last three years. Essentially, major league and minor league strikeout rates have converged into one:

So how can minor leaguers with below average K/PA rates make the jump to the majors and expect their strikeout rates to suddenly reach heights that will drive further increases in the major league strikeout rate?

Based on the data, such a scenario seems very, very unlikely. Instead, the next wave of "incoming" players seem likely to carry with them strikeout rates that are statistically overmatched by those of established and outgoing players, whose strikeout rates are at historically record highs.

As such, look for a reversal—or at least a flattening—of the leaguewide strikeout rate in 2015, and set your category targets accordingly.

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  For more information about the terms used in this article, see our Glossary Primer.