Complete Game Loss

Popular Metric Trends Through the Lens of Percentage Change

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Earlier this week I read a really interesting piece on Fangraphs by Brendan Gawlowski relating to league-wide batting averages. Gawlowski remarked that the league’s cumulative batting average, through just a few days ago, was just .232. He remarked that, “In fact, it’s the lowest since at least the turn of the twentieth century. The .232 mark is five points worse than the league hit in 1968, when Bob Gibson spun a 1.12 ERA, only one American Leaguer managed to hit .300, and nearly a quarter of the season’s games ended in a shutout.” Each one of those facts is pretty incredible.

The most obvious culprit for the persistent declining trend of batting averages are strikeouts. 2021 has started off with more of the same: at the time of Gawlowski’s article, pitchers were striking out 24.6% of the batters they faced, up 1.2% from 2020. While 1.2% may not seem like such a drastic figure, Gawlowski tips his cap to Marc Webster for noting that it represents a considerable uptick in the percentage change across seasons.

And with that, I have arrived at the point of this post. Very simply, I wished to examine changes in popular metrics across seasons, not in their raw forms, but rather in terms of percentage changes, season over season. And that’s about it. What follows are 8 visualizations depicting seasonal percentage-based fluctuations of oft-cited metrics dating back to 2000. Of course, in the case of changes from 2020 to 2021, data are incomplete as it is still just April.

One of the metrics referred to in the aforementioned Gawlowski piece was BABIP, an important component to any given player’s (and thus league-wide) batting average. As Marc Webster (and by extension Gawlowski) noted, the percentage change in BABIP from 2020 to this point in 2021 is larger than any season over season change in the past twenty years.

From 2020 to this point in 2021, league-wide BABIP has dipped >3% season over season to .282.

Three metrics that can easily go hand-in-hand, in addition to being explicitly linked to strikeouts (K% being another hot topic in MLB), include In-Zone%, SwStr%, and Contact%. Season to season percentage changes for those three metrics come next.

Data are available for this metric only dating back to the 2002 season. In the past 19 seasons, dips in In-Zone% have been both more numerous as well as greater in magnitude than rises.
Data are available for this metric only dating back to the 2002 season. In maybe the most relevant chart in this post, SwStr% has increased, season over season, between 1.06% and 4.55% each season for the past 10 seasons leading up to 2021. To this point in 2021, that trend continues.
Data are available for this metric only dating back to the 2002 season. It should come as no surprise that Contact% has suffered just as SwStr% has risen; Contact% has dipped to varying degrees for 9 straight seasons and is on track to do so for a 10th straight year.

Some teams have struggled particularly with the above metrics. The Reds, Rangers, and Rays all have Contact% figures between just 72.0% and 72.1% to this point, representing the worst in baseball. Meanwhile the Tigers, Reds, and Orioles have team SwStr% of 13.6%, 13.4%, and 13.2%, respectively; the three highest such percentages in baseball this season.

Meanwhile, the rise of pitch counts, load-management, deeper bullpens, and openers have all compounded to impact the number of starters’ innings pitched as a percentage of all innings pitched. Below is the seasonal change in starting pitcher innings pitched percentage (a regrettable mouthful of a sentence).

Starting pitchers have been throwing fewer innings than ever, throwing a declining share of innings for six straight seasons. However, to this point in 2021, there has been a reversal to that trend.

Along with strikeouts, home runs make up another key component of the three true outcomes, which increasingly make up greater shares of plate appearance outcomes. As such, home run to fly ball ratios (FB/HR) are closely tracked. This metric has vacillated considerably over the last decade in particular, with a notable spike in the 2019 season and discussions of the juiced ball.

The HR/FB ratio rose from 12.7% in 2018 to 15.3% in 2019, equivalent to a 20.47% rise and the largest such spike (or dip) in the past 20 years. To this point in 2021, HR/FB is down ~10%.

To wrap up, below are charts simple for BB% and K% rates. Differing trends are clear in juxtaposing the two visualizations: for BB%, rates have dipped and risen with similar frequencies and magnitudes (the BB% in 2000 was 9.6% and 9.2% in 2020) whereas league-wide K% has not dipped once since the 2006 season. That stretch encompasses a lot of years for even slight K% bumps to compound; K% was 16.5% in 2000 and 23.4% in last year’s campaign.

Alternatively rising and falling, league-wide BB% has stayed roughly constant over the last two decades.
Climbing between 0% and 6.45% annually from 2006 to 2021, K% has risen from 16.8% (2006) to 24.5% (to this point in 2021), good for a 45.8% rise over that period.

Finally, in order to provide some semblance of context for these percentage changes, the raw seasonal percentages of all the above metrics are featured in the chart below. These figures come right from FanGraphs’ leaderboards so, for a more complete (and interactive) set of metrics, you can also just head over to their leaderboards.

Viewed all at once, it’s easier to glean some overarching trends. In the past half decade, pitchers have left the zone while swinging strikes, home runs, and strikeouts have spiked. Whether MLBs response to these particular trends is to deaden the baseball even more (as it has aimed to do this year) or to move the mound back, it is clear that players, fans, front office personnel, and MLB executives alike are carefully watching how things will continue to change from here.

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