What Makes a Strikeout-Rate Breakout Last?

The emergence of Gerrit Cole as a top-10 starting pitcher and top-three pitcher for strikeouts was one of the more notable developments in fantasy in 2018. As unexpected as Cole’s emergence may have been, he did have a history of being an above-average strikeout pitcher, particularly in his second and third season with the Pirates. Just as noteworthy was a cohort of starters who catapulted themselves well above the major league norm for strikeout rate after having been average or below-average strikeout pitchers prior to this past season.

The top five gainers in strikeout rate among this group are listed in the table below. At the very top is Blake Snell, whom I featured in my recent analysis of walk-rate breakouts. Snell, along with Patrick Corbin, German Marquez, Derek Holland and Mike Foltynewicz, all recorded strikeout rates that were more than six percentage points higher than their 2017 marks, all of which had been near or below the major league average. Of these pitchers, only Holland previously had a season (min. 90 innings) in which his strikeout rate had been as high or higher above the major league average, and that occurred back in 2013.

All five starters, then, could be considered to have had a strikeout rate breakout in 2018. Just as I had established that walk-rate breakouts have been rare over the past 10 seasons, so too are strikeout-rate breakouts of this magnitude. In comparing breakouts going back to 2009, I converted the K% of every pitcher who had thrown at least 90 innings in a season to an index based on that season’s major league average for K%. I used this indexed K% since the average MLB strikeout rate has had some large season-to-season jumps over the past 10 years. Looking only at pitchers who were below the average MLB strikeout rate in the first season of a three-year period, there were only 45 cases of a year-to-year increase in indexed strikeout rate of at least 20 percentage points. To put this in perspective, Foltynewicz’s indexed strikeout rate increased from 95.8 percent of the MLB average in 2017 to 122.0 percent in 2018, for an increase of 26.14 percentage points.

In other words, just 4.5 percent of seasons in which a pitcher was below the MLB average at getting strikeouts was followed by a season in which that pitcher had increased their indexed strikeout rate by at least 20 percentage points. Within this selective group of cases, only 16 had an indexed strikeout rate that was at least 20 percentage points better two seasons later. This means that less than two percent of all seasons in which a pitcher was below average in K% are followed by a sustained two-year K% breakout of this magnitude.

Marquez, Holland and Foltynewicz have already made it through the first hurdle. They are part of the elite 4.5 percent who improved their indexed K% by at least 20 percentage points. (Technically, Snell and Corbin aren’t part of this group, as Snell was 0.9 percentage points above the MLB average and Corbin was at exactly the MLB average in 2017.) But how can we know if these pitchers can extend their strikeout-rate breakouts into 2019?

Some could follow the path forged by Trevor Bauer over the last two seasons. From 2016 to 2017, Bauer’s K% jumped from 20.7 to 26.2 percent, and then it surged again in 2018 to 30.8 percent. That represented an initial indexed K% increase of 23.2 percentage points followed by a year-to-year increase of 16.8 percentage points. Bauer didn’t merely maintain the gains from his breakout, but rather tacked on to them substantially.

Complete regression is also a possibility, as we see from the example provided by Chase Anderson this season. From 2016 to 2017, Anderson bumped up his K% from 18.6 to 23.4 percent, only to see his K% go all the way down to 19.9 percent in 2018. While his 2018 mark was still more than a full percentage point above his pre-breakout rate from 2016, after adjusting for the increase in the average MLB rate, we discover that Anderson gave back 95 percent of his gains.

Bauer and Anderson not only represent two divergent paths that this year’s breakouts could take, but they are also representative of the ways in which those paths diverge. The graph below shows each of the 45 cases in which a pitcher attained a year-to-year increase in indexed K% of at least 20 percentage points since 2009. It displays the year-to-year change in indexed K% — which, going forward, I will refer to as the change from Year 1 to Year 2 — on the x-axis. The change in indexed K% from Year 1 to Year 3 is measured on the y-axis. I am considering anyone who is at or above the 20.0 percent mark on the y-axis to have achieved a sustained strikeout-rate breakout.

The marks on this graph are color-coded for percentage change (not percentage point change) in swinging strike rate. The greener the mark, the bigger the Year 2-to-Year 3 change in SwStr%, and Bauer had the biggest positive change of all of the pitchers represented on the graph. While Bauer is an extreme case, pitchers who gained in SwStr% from Year 2 to Year 3 were more likely to maintain their Year 2 gains in indexed K% than not. Anderson’s SwStr% dropped, and in the vast majority of cases, a decline corresponded with a pitcher losing some or all of their gains. It’s worth noting that in only three of the 45 cases did a pitcher have a worse indexed K% in Year 3 than in Year 1. The track record for breakout pitchers being better strikeout pitchers in Year 3 than in Year 1 — even if it’s just a little better — is solid.

Only two pitchers in this pool experienced a Year 2-to-Year 3 gain in average fastball velocity of at least 1 mph, so it’s hard to conclude that throwing harder helps one to sustain a K% breakout. What is clear from the graph below, which is color-coded for Year 2-to-Year 3 mph change in average fastball velocity, is that losing velocity in Year 3 made it less likely that a pitcher would retain their indexed K% gains from their breakout season. Five pitchers did manage to increase their indexed K% by at least 20 percentage points from Year 1 to Year 3 while losing velocity in Year 3, but a disproportionate number of velocity-decliners fell short of the 20-point mark. Anderson was one of those, while Bauer increased his average fastball velocity in Year 3.

Getting more frequent whiffs and avoiding a drop in fastball velocity appear to increase a pitcher’s chances of maintaining a strikeout-rate breakout, but throwing more strikes does not seem to make a difference. Of the 45 cases in the pool, 19 featured an increase in Zone% from Year 2 to Year 3, but only six of those (or 31.6 percent) had a Year 1-to-Year 3 increase in indexed K% of at least 20 percentage points. A similar proportion (nine of 26, or 34.6 percent) of the pitchers who decreased their Zone% from Year 2 to Year 3 maintained a 20 percentage point increase in indexed K% from Year 1 to Year 3. Once again, Bauer and Anderson exemplified the general trend, as there was not a major difference in their percentage change in Zone%, even though they trended very differently in terms of strikeout rate.

In looking at pitchers like Corbin and Marquez, who made enormous leaps in their strikeout rates, I have admittedly set the bar high in defining a “strikeout-rate breakout.” Accordingly, the sample of three-year trends that meet the criteria for a sustained strikeout-rate breakout over the past nine seasons is quite small. Nonetheless, we can take away that breakouts of a slightly smaller magnitude than those experienced by Holland and Foltynewicz have been rare, but that within this small group, increases in whiff rate have been positively associated with sustained breakouts and decreases in average fastball velocity have been negatively associated. If, come mid-May, members of our 2018 breakout cohort are seeing an uptick in their SwStr% and/or approximating the average fastball velocity they had at a similar juncture this past year, we can assume with some confidence that they are on their way to sustaining the gains they made.