With the year winding to a close, Baseball Prospectus is revisiting some of our favorite articles of the year. This was originally published on July 15, 2015.

When people think back to Game Seven of last year's World Series, the memories most likely to stand out are Madison Bumgarner's masterful five innings of relief and the debate around whether Royals third-base coach Mike Jirschele should have sent Alex Gordon home with two outs in the ninth inning.

However, there was another moment that wasn't aired nearly as often as the Gordon play that was arguably more pivotal in deciding Game Seven. With a runner on second and one out in the fifth inning, Nori Aoki sliced a slider from Bumgarner down the left-field line. Off the bat it seemed like a sure double and one that would have evened the score at three runs apiece. But seemingly out of nowhere, Juan Perez was there to make the running catch.

Perez was in as a defensive replacement for Travis Ishikawa, but he was also perfectly positioned to make the play. But that was Aoki. As far as outfield "shifts" go, the Giants heavily shading their outfield the other way against the slap-hitting Aoki is as close to predictable as you're going to get.

On Wednesday, Jason Kipnis hit a ball similar to Aoki's from last October, albeit in a much lower leverage situation. Playing the role of Juan Perez was Colby Rasmus.

Rasmus plays for the Astros and the Astros received a lot of attention last year for their extreme usage of infield shifts, so it should come as hardly a surprise that they appear to be applying that aggressiveness in infield shifts to their positioning in the outfield.

Take a look at where the Astros were playing various Indians hitters during last week's series at Progressive Field.

Michael Brantley

David Murphy

Yan Gomes

Michael Bourn

These aren't the extreme shifts in alignments we've become accustomed to in the infield, but they are also more radical than an outfielder taking a few steps to his left or right. Regardless of whether you want to call this "shifting" or "shading," the data and scouting reports clearly indicated to the Astros that certain hitters were worthy of being defended aggressively on balls hit in the air.

Aggressive outfield positioning isn't a completely new development for Houston:

The Astros Freddie Freeman shift: Infield shifted to pull, outfield opposite field. Never seen anything like it
— Hashim (@Hashim305) June 25, 2014

There, Jose Altuve's positioning in short right field against Freddie Freeman allowed George Springer to shift into Minute Maid Park's spacious right-center-field gap.

We've seen the occasional case of extreme outfield shifting based on a park's dimensions in the past. As evidenced in last week's series in Cleveland, the Astros are inclined to heavy shading based on the hitter's batted-ball profile regardless of the ballpark, but at home, shading their right fielder into the gap against left-handed batters has even more benefit. Here's an example of Springer taking away a normally surefire extra-base hit destined for the right-center gap

and another of that same positioning making what could have been a difficult play look routine.

But of course there's a heightened risk associated with outfield shifting when comparing it to stacking one side of the infield. If a ball scoots through the bare side of the infield, the consequence is the difference between a hit and an out. In the outfield we're not only talking about the difference between a hit and an out but also about the potential for additional bases, with the most obvious case being on balls hit down the right-field line by left-handed hitters that could turn into triples.

This risk is exacerbated with runners on base (in particular with a runner on first), with the massive pockets of empty space in the outfield effectively representing the opposite of a no-doubles defense. Take the extreme outfield shift against Yan Gomes from earlier. A ball hit down the left-field line into the corner is almost never going to result in a triple for a right-handed batter. But with a runner on first, shifting the left fielder heavily towards the gap could easily be the difference between holding that runner at third and having him score.

A situation of that type nearly bit Houston during last Wednesday's game when they had one of their more extreme outfield shifts on against David Murphy. (The screenshot of Houston's alignment against Murphy from earlier was prior to this hit.)

With a faster runner, a more aggressive third-base coach, or even a less favorable bounce off the wall for Preston Tucker, this would have been a run on a ball that Tucker could have had a chance to limit to a hard-hit single had he been playing a more standard right field. But optimal positioning is all about playing the percentages, and a look at Murphy's spray chart makes clear why the Astros were playing him to hit the ball to the opposite field in the air.

This is obviously a quick-and-dirty snapshot at Murphy's hitting tendencies in the aggregate. The Astros were surely adjusting their defenders against Murphy based on additional factors, such as the type of pitcher, the count, and the ballpark. However, what is clear is that Murphy hits the majority of his balls in the air to center and left field, which is why Colby Rasmus and Jake Marisnick were both so heavily shaded toward that side of the outfield. Tucker playing in the right-center gap was simply necessary to avoid massive space between himself and Marisnick.

Defensive shifts in the infield have become common in today's baseball landscape, but outfield positioning has received far less attention. One reason is that outfield alignment is more difficult to track through broadcasts as companies like Baseball Info Solutions and Inside Edge have been doing with infield shifts.

Second, aggressive positioning in the outfield lags behind its infield counterpart in extremity, so it's less likely to grab the attention of the broadcast booth or the viewers at home. A center fielder shading to his left or right, or taking a few steps deeper or shallower, against a particular batter has been in practice for decades, at least. The same goes for no-doubles defenses. However, it's not as if teams are going to start stacking one side of the outfield with all of their outfielders.

Visually, infield shifts are more extreme than outfield shifts, but positioning outfielders more efficiently is something that could have a larger effect because of the effect on extra-base hits. There are likely fewer shift candidates in the outfield than the infield, as the distribution by direction of balls hit to the outfield is typically more spread out than on balls hit in the infield. But in cases where the data indicates that outfielders should be heavily shading one way or another, teams stand to gain an even greater benefit from playing the percentages than they do in the infield.

Of course it's not as if the Astros are the only team using the ever-increasing data available to teams to optimally position their outfielders. In fact, last March, Anthony Castrovince of wrote that the Pirates "are convinced that infield shifts were so instrumental in the progress of their pitching staff in the organization's first winning season in 21 years that they plan to expand their use of the shift this season to incorporate more aggressive outfield positioning, as well." It certainly wouldn't surprise me if other teams that have popularized the use of infield shifting, such as the Rays, Orioles, and Yankees, were also aggressively positioning their outfielders.

At its core, shifting is really just about maximizing defensive efficiency. Over the past few years teams have recognized this and become better at optimally positioning their infielders. It seems that the Astros are one team that is starting to do the same in the outfield.

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I don't think that trading a few outs for a bunch of doubles and triples is the best of ideas.