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Tanner Roark’s Z-Swing%, and Related Observations

Although the Nationals had a disappointing 2013 season overall, Tanner Roark (RHP) was one of their more pleasant surprises. The Nats brought him up in August, as injuries and performance problems created openings for several pitchers in their minor league system.

While Taylor Jordan also performed well, I think it’s fair to say that Roark had the most impressive and intriguing debut for the big-league team. Roark accumulated excellent “traditional” stats, and he did so at least in part by exploiting an unusual but highly effective talent: making batters not swing at good pitches. This post explores Roark’s story, and opens up the question of how his distinctive forte, zone-swing rate, contributes to effective pitching.

To recap, Roark finished 7-1 with a 1.51 ERA over 53 2/3 innings. He allowed only 1 home run in total, or 0.17 home runs per 9 innings; and the league batted .197 against him (– “batting average against” or “BAA”). The Nationals’ ace, Stephen Strasburg, allowed 0.79 home runs per 9 innings, with a BAA of .205. Roark was comparable in BAA to Strasburg, and much, much better at preventing home runs.

Of course, Strasburg reached his figures in 183 innings of pitching as compared to Roark’s 53 innings of pitching. This is what is sometimes described as a smaller sample. But we should not discount Roark’s performance too quickly. His 53 innings involved five starts and nine relief appearances, and a total of 12 appearances with at least two innings pitched. This is considerably more than, say, one start and no relief appearances. Roark played for the Nationals for the last two months of the season. His stint in the majors last year was substantial enough, I think, to merit serious interest.

Roark’s 2013 performance was surprising in part because of his pedigree. In 2012 Roark was 6-17 as a starter in Triple-A, pitching for the Nationals. His 2012 ERA in Triple-A was 4.39 (although his FIP [Fielding Independent Pitching rating] of 3.85 was better). Providing more background, Adam Kilgore wrote in September 2013 that

Roark has never been regarded as a star or a significant prospect. In 2008, the Rangers drafted him in the 25th round. The Nationals acquired him and another minor league pitcher for Cristian Guzman at the 2010 trade deadline. Last winter, the Nationals left Roark unprotected from the Rule 5 draft for the second straight year. They invited him to major league spring training this year, and shipped him out in the first round of cuts.

(Washington Post, Nationals Journal, 9/17/2013; http://www.washingtonpost.com/blogs/nationals-journal/wp/2013/09/17/tanner-roarks-incredible-start-built-on-command-feel-for-pitching/)

Roark’s 2013 performance was also surprising because, with a fastball averaging 92.6 mph, he had good but not overwhelming velocity.

Going back to FIP and similar topics, another reason why Roark’s 2013 performance was surprising was because of some relationships between his statistics. For instance, although his 2012 Triple-A ERA (4.39) was higher than his 2012 Triple-A FIP (3.85), this relationship reversed itself last year in the majors, with Roark posting a 1.51 ERA and a 2.41 FIP. In addition, his xFIP (“expected Fielding Independent Pitching”) was 3.14, significantly higher than the FIP.

“ERA < FIP < xFIP” spreads of this size are not unheard of, but they are rare, especially when your ERA is less than 2.00. In fact, ERA < FIP < xFIP distributions of this type suggest that you are identical to Clayton Kershaw (1.83 ERA / 2.39 FIP / 2.88 xFIP) and that you have just signed a contract worth 215 million dollars!

These observations about Tanner Roark’s performance and pedigree raise several questions:

How did he perform so well in 2013?

What is going on with his ERA<FIP<xFIP distribution?

What can we say about his future performance?

Taking a quick initial look at the ERA<FIP<xFIP distribution, a “negative” delta between ERA and FIP is often attributable to the pitcher having a low Batting Average on Balls in Play (BABIP). Roark’s BABIP was indeed very low, at .243. (Kershaw’s was .251).

Also, although this might sound odd, Roark’s extremely low HR rate (0.17 per 9 innings) pushed his ERA below his FIP, even though home runs are a fielding-independent matter. Roark was fine (league average or better) on the other FIP elements — walks, K’s, HBP’s. But combining these normal-range statistics with his homer rate produces a compromise number and some information loss.

Turning to xFIP, this calculation substitutes out the pitcher’s own homer rate for the league average homer rate. As we might expect, the league average homer-rate was much higher than Roark’s, and this explains the FIP < xFIP delta, while also contributing to the delta between his ERA and his xFIP.

These observations tend to intimate that some of Roark’s statistics are not likely to repeat themselves. Before turning to the “future performance” question identified above, I want to look more at the first question of trying to understand Roark’s 2013 success. There are aspects of Roark’s pitching last year which suggest that his strong performance numbers were not an accident, and that his apparent prowess is not simply overmagnified by the small prism of his innings total.

The first statistic of interest is that Roark was seriously good at throwing pitches in the strike zone which batters did not swing at. This is the Z-Swing% statistic recorded on FanGraphs and other places. Roark’s Z-Swing rate in 2013 was 54.8% (per Baseball Info Solutions [BIS]), or 55.9% per PITCHf/x. This means that batters only swung at Roark’s pitches in the strike zone about 55% of the time.

(BIS and PITCHf/x converge around 55% for Roark’s Z-Swing%. These systems actually diverge, or report different percentages, for some other stats which are not independent of Z-Swing%. Although this is interesting, the differences do not materially affect our evaluative questions. I will cite the BIS plate discipline statistics throughout and compare them to PITCHf/x at various points below).

The complement of Z-Swing% is what I will call “Z-pass” — the phenomenon of non-swings on pitches in the strike zone. Tanner Roark’s Z-pass rate last year was 45% — batters passed on about 45% of his pitches in the strike zone.

This was a very high Z-pass rate. In fact,

A high Z-pass rate is indicative of several good pitching qualities. Z-passes are good because they mean that batters are laying off a higher number of pitches which damage their cause and advance the pitcher’s cause. A high Z-pass rate indicates that the pitcher is accumulating strikes while maintaining an atypically lower risk of allowing a hit. (This is true if the pitcher is hitting the strike zone at a reasonable rate. More on this below). Tactically speaking, the Z-pass is the best outcome on the swing v. strike zone matrix below.

In Zone

Out of Zone




No Swing



Swings on pitches in the zone and out of the zone can lead to hits, and worse. By contrast, if we assume that non-swings in the zone lead to strikes, the Z-pass simply constitutes a good outcome for the pitcher.

How often did Roark throw strikes? In 2013 Roark hit the strike zone 47.7% (BIS) of the time. This was about 3 percentage points ahead of major league average (44.9%). 3 percentage points comes out to about one standard deviation above average. (PITCHf/x reports a higher league-wide strike-zone rate — 49.4% — and a higher strike-zone rate for Roark as well, at 53.8%. PITCHf/x appears to have a larger strike zone than BIS).

It therefore appears Roark was exploiting his elite Z-pass rate often enough for it to be useful, and indeed for him to have an advantage over hitters. Roark accumulated strikes at a good rate; and, by strongly suppressing swings at pitches in the zone, he lowered the risk of allowing a hit. It appears this dynamic was a main factor in Roark’s success in 2013. That’s part of the answer to our “How did he perform so well” question.

Another factor which stands out from Roark’s strike-zone data is that he threw first-pitch strikes 70.6% of the time. This tied for third in major-league pitchers with at least 50 innings in 2013. Consistently gaining an initial advantage over hitters, and doing so at an elite rate, was another main factor in Roark’s success.

Other discussions of Roark have cited his command, his aggression, and an improved mental approach. Going back to Adam Kilgore, he writes:

Roark’s ascension began last season, when he told himself he would not allow his temper to control him on the mound. He would not the things out of his control – fluky hits, errors, whatever – distract him. He would throw strikes. He would be confident. He would attack, above all else.

“I feel that last year is when I had my, I guess, mental turnaround,” Roark said. “That was the biggest thing for me.”

(Washington Post, Nationals Journal, 9/17/2013; http://www.washingtonpost.com/blogs/nationals-journal/wp/2013/09/17/tanner-roarks-incredible-start-built-on-command-feel-for-pitching/)

We can certainly see command at work in Roark’s low homer rate, and his low walk rate (5.4%). We can see both command and aggression at work in his first-pitch strike rate. Roark’s league-leading Z-pass rate substantiates the command/aggression understanding of his performance, and also adds to this understanding.

A pitcher who suppresses swings on pitches within the zone is presumably hitting unattractive parts of the zone, but he may also be throwing in-zone pitches which do not present to hitters as strikes. This sounds like a pitcher on whom it is difficult to make good contact. This is a third idea, beyond Z-pass rate and first-pitch strike rate. One way, however, to be averse to good contact is to be a high Z-pass pitcher.

Being a high Z-pass pitcher does not entail being a high strikeout pitcher. Roark’s strikeout rate was only one percent below major-league average (again, among pitchers with 50 innings and up). Of course, on other measures, like ERA, Roark was much better than league average. I think that connecting Z-pass rate with suppression of good contact can help us understand why.

Z-passes represent hittable pitches – pitches in the zone – which were not hittable enough to induce a swing. Poetically speaking, Z-passes involve real visual ambiguity: since they end up in the strike zone, they can’t look that bad; but they do not look good enough to induce a swing.

How well does this characterization actually apply to Roark’s pitches? On this question, we have the following from the Atlanta Braves:

“He wasn’t missing with any pitches over the plate, it seemed like,” said Braves catcher Gerald Laird. “When he was going away, he was throwing that little two-seamer back door, when he was coming in he was running that two-seamer in on your hands, and he had that little slider working.

“Tonight it seemed like he was hitting his spots and wasn’t making any mistakes. I know (Freddie Freeman) was saying he was starting it at him and running it back over. When he’s doing that it’s hard to pull the trigger.”


Of course, these descriptions of visual ambiguity — or of evidence which shifts within a fraction of a second — presumably apply to all or most of a high Z-pass pitcher’s offerings, not just to his pitches in the strike zone which do not elicit a swing. The image that emerges is of a player whose whole volume of pitches is tough to react to in a manner that creates good contact.

Roark was actually pretty good at inhibiting contact of any kind, especially on pitches within the strike zone. However, a look at his contact numbers does not immediately confirm this interesting and important point. As we see in the table below (from BIS by way of FanGraphs, again looking at 50+ IP), many of Roark’s contact rates were actually above league average, sometimes by more than one standard deviation.




















MLB (50+ IP)










std dev










Before turning to contact rates, you will have noticed that this table also gives us a look at how Roark’s Z-swing rate compared to the rest of baseball. According to BIS, Roark was 3 standard deviations above average on a positive pitching statistic which is completely independent of fielding. He was two standard deviations (56% Z-Swing%, as opposed to 63% league average) ahead according to PITCHf/x — this is still pretty good for a former 25th-round pick! Some other observations:

(PITCHf/x condenses this gap, in much the same way that it condenses the gap between Roark and MLB on Z-pass. PITCHf/x reports Roark at 52.2% contact on all pitches within the zone, and MLB at 54.6%. Thus, if we switch from BIS to PITCHf/x, Roark’s contact rate goes up, and MLB’s goes down.

However, as noted above, PITCHf/x appears to be working with a larger strike zone than BIS (MLB-average Zone% of 49.3 vs. MLB-average Zone% of 44.9). This point complicates Roark’s apparent movement back towards league average. In brief, the fact that Roark’s swing rates go up — while the MLB average goes down — on larger renditions of the strike zone may be a testament to his effectiveness, rather than a knock against it.

The standard contact rates reported by BIS and PITCHf/x do not do a good job of communicating how well a pitcher actually prevents contact, because these contact rates only look at swings. Since you can suppress contact by suppressing swings, multiplying the contact rate by the swing rate provides a better view of how a pitcher is actually doing along this dimension. Despite a “zone-contact” rate which was higher than league average, Roark was very good to excellent at suppressing contact within the strike zone.

We are exploring a clue provided by Roark’s excellent Z-pass rate that Roark was good at inhibiting solid contact. This clue was supported by our look at Roark’s contact rates, which indicate that he was pretty good at suppressing contact flat out. The idea that Roark’s pitches were visually ambiguous enough to limit good contact receives further confirmation from his batted-ball statistics. In addition, looking at these statistics (2013, 50+ IP) will bring us around nicely to the question of how well Roark might sustain his performance in future seasons.
















MLB (50+ IP)








Std dev








Roark’s ground-ball, fly-ball, and infield-fly rates combine to indicate a strong bias against good contact. Roark had a somewhat high line drive rate, and, admittedly, line drives are a form of good contact. For instance, I suspect it’s unusual to have a somewhat high line-drive rate and a markedly low BABIP. Roark’s line-drive rate provides one specific indication that his BABIP is due to increase. However, a low line-drive rate is not entirely at odds with the idea that a pitcher is suppressing good contact — especially if we are thinking about home runs. Since most line drives are not home runs, a slight tendency towards line drives is a small but genuine homer-prevention measure.

In this way, Roark’s line drive rate coheres with his ground-ball, fly-ball, and infield-fly rate statistics. All of these rates, and especially their combination, suggest a low-homer pitcher. Why didn’t Roark give up a lot of home runs? Well, he got a lot of grounders and infield flies, while limiting his fly balls overall, and he gave up a somewhat high proportion of line drives. It is very plausible to suppose that Roark’s extremely low HR/FB rate overshoots the anti-homer bias suggested by his other batted-ball rates. Equally, however, the other rates tell a clear enough story that a low homer rate is not at all a surprise. Roark was very good at inhibiting good contact.

How will he do in the future? A nice way to frame this question is in terms of Roark’s ERA, FIP, and xFIP numbers mentioned earlier. And, leading up to that, I think it’s helpful to assess the respective importance of two things: (1): the overall coherence of Roark’s 2013 statistics; and (2) the sample sizes in which they were achieved.

In terms of coherence, Roark’s statistics tell a consistent story:

I could be wrong about this, but I do not see relationships among Roark’s 2013 statistics which point to trouble looking ahead. These statistics tell a consistent story of effectiveness. You can focus on his low swinging-strike rate if you like, but this rate was consistent with Roark being at least one standard deviation (two sd’s according to BIS) better than average on limiting contact within the zone.

In addition, there are pockets within Roark’s portfolio where some stats are very good and others are even better, like the HR/FB rate relative to Roarks other batted-ball statistics. However, this type of overshooting is a good problem to have. To the extent that the non-harmonic components of Roark’s statistical portfolio are extremely good statistics, this relates to the issue of our expectations for future years. A version of Tanner Roark based on 2013, but without the extra anti-homer overshooting, would still be above MLB-average.

As noted above, Roark only pitched 53 innings, and that’s a much lower total than what a starting pitcher would typically accumulate over a full year. Although we intuitively regard this as a small sample, it does not follow that Roark’s performance is without predictive value. As is often pointed out on the pages of FanGraphs, statistics stabilize, or acquire predictive value, at different thresholds (http://www.fangraphs.com/library/principles/sample-size/). Generally speaking, fielding-independent stats stabilize more quickly for pitchers than fielding dependent stats; this is a helpful point in assessing the forward relevance of Roark’s 53 innings.

Some of Roark’s relevant statistics are above their stabilization thresholds. Roark allowed 153 balls in play (BIP), which puts him above the stabilization points for groundball rate and flyball rate:

70 BIP: GB rate

70 BIP: FB rate

Roark faced 204 batters, which is above the stabilization points for walks and strikeouts:

70 BF: Strikeout rate

170 BF: Walk rate

However, Roark was league-average in K’s and was “only” one standard deviation above average in walks; these numbers are not as good as Roark’s plate discipline statistics like Z-pass and suppression of contact within the zone. So it’s not clear whether Roark reached the stabilization points for key parts of his performance.

But this is more or less where I will have to leave it. Figuring out the stabilization point for Z-pass is beyond the scope of the present study. Indeed, my post has probably pushed us to near overload regarding things that we ever wanted to know about Tanner Roark! By the same token, it’s not clear that learning more about Roark’s statistical profile would shift our opinion much about his prospects for future performance. This is what I think we have to consider:

In an intuitively small sample size, Roark put up a consistent portfolio of excellent fielding-independent stats: on limiting zone-swings, limiting contact in the zone, and limiting good contact. Very broadly, the size of a sample has to be balanced with the consistency of the evidence within it. Just imagine watching a one-round boxing match in which one competitor knocks the other one down three times. This is a small sample which tells a very compelling story about the respective abilities of the boxers. Roark’s sample size is larger, of course, and his performance was not as dominant. Nonetheless, his limited 2013 season is packed with a lot of positive indicators.

Here are a few final comments about what Roark might do in the future, framed in terms of his ERA, FIP, and xFIP:








MLB average (50+IP)




standard deviation




As we discussed above, the delta between Roark’s ERA and FIP is primarily a matter of his low BABIP and his very low homer rate. Although Roark’s BABIP will probably go up, there are signs he may be better than average at suppressing hits: he showed a tendency to induce ground balls and infield flies; the latter especially inhibit BABIP.

Roark’s very low homer rate pulls down both his ERA and his FIP. Although his .17 homers per 9 innings will almost certainly go up, there are signs he may be better than average at suppressing home runs…signs which are distinct, that is, from his one homer allowed in 53 2/3 major league innings!! Roark’s tendencies toward ground balls, infield flies, and line drives are all anti-homer measures. These tendencies flow, by hypothesis, from his ability to inhibit good contact by throwing visually ambiguous pitches.

The most eligible view by far is that Roark will regress towards league average in future years. But accepting this view should not deprive us of optimism. Roark could go back at least one standard deviation on each of the ERA-like measures and still be at league average or better than league average. That’s a good position for any pitcher. It’s a great position, albeit a paradoxical one, for a pitcher who is currently slated to compete for no better than the 5th spot in the Washington Nationals’ 2014 starting rotation!! Suffice to say I think that Roark ought to receive full consideration for the opportunities available to him.