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The Lucky and the Strong: Astros Starting Pitching is Exceeding Expectations

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Astros starting pitching has performed better than advanced pitching stats would indicate. Much of this may be luck, but fortune favors the prepared mind.

Houston Astros v Detroit Tigers Photo by Gregory Shamus/Getty Images

Dominant on the Scoresheet

Before the season began, the Astros starting rotation depth was in question. Over halfway through the season, that question has been put to rest. The rotation depth is more than adequate; it’s now a strength. The Astros have enough quality starting arms that Cristian Javier has been shifted into relief to help a struggling bullpen, and the Astros have employed a six-man rotation.

Entering July 21, the Astros starting pitchers rank first in the American League in WHIP (1.12) and are second only in ERA at 3.36 to the White Sox’ vaunted rotation (3.29). They are tied for first in pitcher wins with 40, and have allowed the fewest runs in the league (205), despite throwing the second most innings in the league at 530.2 IP (Oakland starting pitchers have pitched a whopping 559 innings)

2021 Houston Astros Starting Pitching Stats

Pitching Stat Value AL Rank
Pitching Stat Value AL Rank
ERA 3.36 2
Wins 40 1
WHIP 1.12 1
Runs allowed 205 1
Innings Pitched 530.2 2
Through July 20, 2021 @mhatter106, data from Fangraphs


This is dominance. And one that has been sorely needed, with Houston relief pitching faltering frequently this season.

A closer look into advanced pitching stats, however, suggests that Astros starting pitching haven’t actually been pitching as dominantly as scoresheet results would indicate.

In FIP (Fielding Independent Pitching), xFIP (Expected FIP), and SIERA (Skill-interactive ERA), Astros starting pitching ranks closer to the middle of the pack. The same is true of their strikeout rate and walk rate.

2021 Houston Astros Starting Pitching Advanced Stats

Pitching Stat Value AL Rank
Pitching Stat Value AL Rank
FIP 3.92 5
xFIP 4.08 5
SIERA 4.16 7
K% 23.7% 6
BB% 7.9% 7
Through July 20, 2021 @mhatter106, data from Fangraphs

These ranks are still good. The Astros still rank within the top half of the league in all the above categories. But these don’t look like the marks of a valedictorian, but more like those of a biker near the front of his peloton.

In fact, no AL team has as large a negative discrepancy between their ERA and their FIP, xFIP, and SIERA as the Houston Astros.

Difference between Astros Starting Pitching ERA and FIP/xFIP/SIERA

Compared metrics Differential AL Rank
Compared metrics Differential AL Rank
ERA - FIP -0.57 1
ERA - xFIP -0.72 1
ERA - SIERA -0.80 1
Through July 20, 2021 @mhatter106, Data from Fangraphs

Why is there such a discrepancy between Astros starting pitching standard stats and their advanced stats?

The Astros starters are stranding runners at a league high rate.

The Astros’ 78.3% LOB% (Left on Base Percentage) is highest in the AL, and well above the league rate of 72.1%.

Houston Astros Starting Pitching LOB%

Year HOU LOB% AL LOB% AL Rank
Year HOU LOB% AL LOB% AL Rank
2021 78.2 72.1 1
2020 70.7 72.3 10
2019 76.3 70.9 1
2018 77.4 71.6 1
2017 74.0 72.1 3
Through July 20, 2021 @mhatter106, Data from Fangraphs

The Astros have actually excelled in stranding runners for the last several years, excluding the pandemic shortened 2020 season.

LOB% generally does regress towards league average, but it is true that there is some control of keeping your LOB% higher. This is usually driven by pitchers with high strikeout rates though. The 2018 and 2019 Astros rotations were led by the two premier strikeout pitchers of the league in Justin Verlander and Gerrit Cole, something the 2021 does not have.

The 2021 Astros starting pitchers may still end the season on the better side of league average, but some regression towards 2020’s LOB% would not be unexpected.

The BABIP Dragon has been particularly kind to Astros starters.

What happens when Astros opponents put a ball in play against Houston starting pitchers? A lot less than what happens against other teams.

The Astros starting pitchers have a BABIP (Batting Average on Balls in Play) of just .258 this year. This is markedly lower than the second lowest team in the league, Detroit at .274, and far lower than the AL mean BABIP of .294.

@mhatter106

BABIP is also something that tends to regress to the league mean. It would make sense to expect a substantial increase in the Astros starting pitching BABIP the rest of the season. . . except for the fact that, like LOB%, the Astros starting pitchers have consistently have also been among the lowest in the league in BABIP.

Houston Astros Starting Pitching BABIP

Year HOU BABIP AL BABIP AL Rank
Year HOU BABIP AL BABIP AL Rank
2021 0.258 0.294 1
2020 0.270 0.288 2
2019 0.270 0.300 1
2018 0.284 0.293 4
2017 0.299 0.297 9
Through July 20, 2021 @mhatter106, Data from Fangraphs

How is it possible that the Astros can keep flipping heads instead of tails at this rate for so long? Luck obviously plays a factor. That’s not really something you can control. Pitching profile (ground ball vs fly ball) will also play a role.

Certainly defense. The 2021 Astros’ defense this year has been consistently excellent. They lead the AL in DEF (Defensive Runs Above Average) on Fangraphs this year with 15.6.

It would stand to reason employment of defensive shifts helps too. No team in the league employs defensive shifts as much as the Houston Astros do. And while with each successive year, the rest of the majors catches up, the Astros still lead the pack.

Houston Astros Defensive Shifts By Year

Year HOU Shift % MLB Shift % AL Rank
Year HOU Shift % MLB Shift % AL Rank
2021 44.7 31.5 1
2020 44.1 34.1 2
2019 49.5 25.6 1
2018 37.3 17.4 1
2017 34.3 12.1 1
Through 7/20/21 @mhatter106, Data from Baseball Savant

But you’ll notice that although major league baseball’s use of the shift as a whole has gone up each successive year, the AL’s BABIP hasn’t seen a corresponding downward correlation.

Minute Maid Park itself may be a factor. Astros starting pitchers have routinely experienced more favorable BABIP at home than away in the past several years. That doesn’t seem to be the case in 2021 where so far their road BABIP is even lower than their home, but if Minute Maid has a tendency to dampen BABIP, but it’s possible that Astros starting pitcher BABIP could have been elevated further.

Houston Astros Starting Pitching BABIP, Home/Away Splits

YEAR HOU Home BABIP AL Home Rank HOU Away BABIP AL Away Rank
YEAR HOU Home BABIP AL Home Rank HOU Away BABIP AL Away Rank
2021 0.267 4 0.248 1
2020 0.254 4 0.287 7
2019 0.264 1 0.276 2
2018 0.272 3 0.296 7
2017 0.287 6 0.322 13
Through July 20, 2021 @mhatter106. Data from Fangraphs

Whatever the reason, and it is almost certainly a combination of many reasons, a consistently low BABIP year in and year out suggests it can’t all be luck. But when you sport a .258 BABIP when the rest of the league averages .294, at least some of it is good fortune. And the coin will land on tails sometime.

The Astros starters have indeed been on the lucky side.

Looking at expected batting stats from Baseball Savant, the Astros starting pitchers have a xBA (expected batting average) of .229 against them and an xwOBA (expected weighted on-base average) of .299. These are great numbers. Both rank 2nd in the AL, behind only the White Sox.

But their actual BA against and wOBA against are even lower than their expected counterparts at .218 and .287 (Also both second in the league). In the AL, only Detroit has a bigger negative difference between their BA and xBA (-.011), and only Detroit and Seattle have a bigger negative difference between their wOBA and xWOBA (-.012).

As the sample sizes increase as the season continues, actual batting stats against ought to more closely mirror the expected batting stats against.

Inspiration and Perspiration

The 2021 Astros starting pitchers have been very good, but the degree of their success has exceeded what advanced pitching stats would suggest. A league high LOB% and a league low BABIP appear to be at the root of this. Some regression of those values is inevitable, but maybe not as much as Astros fans might fear. The Astros have a history of low BABIP and high LOB%, so if it’s all luck, then perhaps the Astros making some of their own luck.

It is said that it is better to be lucky than to be good. Houston Astros starters have been a bit of both.