Originally written on Fangraphs  |  Last updated 10/30/14
The infield fly ball is the second worst outcome for a hitter besides a strikeout. With almost 100% of all popups turning into outs, a hitter, who is prone to skying the ball into the infield, will generate more outs and therefor a lower batting average. Several factors make a pitch more likely to be hit as an infield fly ball, but the key factor is the batter’s mechanics. Background A few initial facts on infield flies balls (IFFB) can help with understanding them. 1. Infield flies (used interchangeably with pop ups in this article) have a direct effect on a player’s BABIP. Here is a plot of the BABIP and infield fly ball percentage (IFFB%) for all players with more than 300 PA from 2002 to 2012. Using the equation generated from the best fit line, a player’s BABIP will drop around 3 points for every 1% point increase in IFFB%. A hitter with a 5% IFFB% will normally have a BABIP around .316 while a hitter with a 15% IFFB% will have a BABIP around 0.286. 2. IFFB% stabilizes fairly quickly. Pizzacutter found that it took 500 PAs for pop ups to stabilize. Derek Carty found that it took about half that number of plate appearances. No matter the exact number, a player IFFB% will stabilize within a single season. 3. Besides stabilizing quickly, the IFFB trait correlates from year-to-year. Here is the plot of all players with over 300 PA in 2011 and 2012 and their IFFB%. Batters who pop up tend to do so the next season. Research Note: In the process of writing this article, I ran many queries on IFFB% and found some data irrelevant to this article. I have included the data in an Appendix at the end of the article for those who can’t get enough numbers. I sorted out some of the conditions that lead to a high number of IFFBs for a hitter. The first bit of useful information is pretty obvious. A hitter is more likely to induce a fly ball on a pitch that doesn’t break downward. Pitch: IFFB% Cutter: 9.3% 4-finger fastball: 8.4% Knuckle: 8.1% Slider: 7.6% Change up: 5.9% Split Finger: 5.3% 2 Finger fastball: 5.0% Curve: 4.6% Sinker: 4.0% The reason the cutter induces so many infield flies is because it moves inside on a hitter and makes contact with the narrow part of the bat. If the ball happens to be a bit on the high side of the thin part of the barrel, a pop up occurs. Or a weak grounder happens if the ball hits on the bottom half. A batter wants to keep the ball on the barrel of the bat. Besides the pitch type, the pitch’s location is important to inducing pop ups. The key location to induce a pop up is high and inside. Here are two tables representing to the IFFB% for various pitches in the strike zone. The area covers the rule book strike zone and is from the catcher’s perspective. Right handed Hitters -1 to -0.5 -0.5 to 0 0 to 0.5 0.5 to 1 3 to 3.5 21% 15% 11% 8% > than 10% 2.5 to 3 14% 10% 7% 5% 6% to 10% 2 to 2.5 8% 6% 5% 4% 5% or less 1.5 to 2 5% 4% 4% 4% Left Handed Hitters -1 to -0.5 -0.5 to 0 0 to 0.5 0.5 to 1 3 to 3.5 6% 11% 18% 24% > than 10% 2.5 to 3 4% 7% 12% 17% 6% to 10% 2 to 2.5 3% 4% 7% 11% 5% or less 1.5 to 2 2% 3% 4% 5% High and inside is by far the best location to induce a fly ball. Just 12% of the all the pitches are in the upper inside zone and the two zones adjacent to it (Note: I will refer to these 3 zones as the IFFB Zone from now on), but these pitches induce 28% of all the pop ups. The problem for hitters is that they just can’t lay off and take a pitch in the IFFB Zone for a ball. While pitches in this area are near the edge of the umpire called strike zone, 73% of all taken pitches in this area are called strikes. With that background, I looked at what makes a hitter more prone to IFFBs. I thought the answer would be easy. Possibly players who swung at more pitches in the IFFB Zone hit more IFFBs. After running way too many tests, I actually found that hitters that swung at pitches in the IFFB Zone where less likely to hit an IFFB. It seems that hitters who can handle the high inside fastball, seem to swing at it. Those hitters who can’t handle the pitch lay off of it. Example At this point, I decided to compare 2 somewhat similar power hitters. One with a high IFFB% and another with a low value. For this comparison, I looked at Buster Posey and Edwin Encarnacion. Name Team IFFB% BABIP BB% K% HR/FB LD% GB% FB% AVG OBP SLG Buster Posey Giants 4% 0.368 11% 16% 19% 25% 47% 29% 0.336 0.408 0.549 Edwin Encarnacion Blue Jays 12% 0.266 13% 15% 19% 18% 33% 50% 0.280 0.384 0.557 Truthfully, I don’t like this comparison, but I will live with it. I wanted two hitters on the opposite sides of the IFFB spectrum. I got that. I got similar HR/FB rates. Both are right handed. Both have similar K% and BB%. At that point, the similarities end. To get players at the extreme ends of IFFB%, some compromises where made. As seen in the above stats, Posey only had a 4% IFFB% compared to Encarnacion’s 12% rate in 2012. Besides their overall stats, here are the results of the balls that each hitter made contact with in the IFFB Zone during this past season: Batted Ball Type: Number (Percentage) Encarnacion Grounder: 14 (29%) Liner: 6 (12%) Fly Ball: 11 (22%) Pop Up: 18(37%) Posey Grounder: 16(32%) Liner: 16 (32%) Fly Ball: 18 (36%) Pop Up: 0 (0%) Posey did not hit one high inside pitch for a pop up. On the other hand, Encarnacion popped up the ball over 37% of the time. The difference in how the pair handles inside pitches can be seen in this run value heat map (Posey’s values compared to Encarnacion). The heat map is from the catcher’s perspective. The contrast is easy to spot with Posey getting better results on the inside part of the plate compared to Encarnacion. To find the difference in how the pair handles inside pitches can be achieved by looking at just few swings each batter made at pitches in the IFFB Zone. Here is the pair swinging at a pitch high and inside that resulted in a fly out to the outfield. Besides the two videos, here are 6 images of each hitter when they swung at a pitch in the IFFB Zone on a 0-0 counts. I only wanted examples when the hitters were not trying to protect the plate, so I limited the data to 0-0 counts. (embiggen) Several traits can be seen that allow Posey to pop up less often. 1. First, Posey does not drop his back elbow as much as Encarnacion and therefor keeps his swing more level. Encarnacion has a natural upper cut to his normal swing, but it gets exaggerated when dealing with pitches high and inside. Since Posey doesn’t drop his back shoulder as much, he is less likely to pop the ball up. 2. Most of Posey’s power comes from his hips (which are facing the pitcher when the ball and bat make contact) and his wrists, that he keeps in. By keeping his wrist close to his body, Posey is able to make contact with the solid part of the bat. Encarnacion keeps his bat away from his body. Since he has the bat further away, he makes less solid contact. A couple of the images show the ball hitting Encarnacion’s bat on the handle. 3. Finally, Posey repeats his swing exactly the same each time. His feet, hips, head and wrists are all in the same location in each image. Encarnacion is all over the place with his mechanics. For example, his feet are in a different position in each image. Posey has found a swing that works for him on inside pitches and he repeats in constantly. Encarnacion just can’t seem to get comfortable on the inside swing and it shows with his results. Not all hitters who hit a high number of IFFB will have the same traits that Encarnacion exhibit or vise versa for hitter who don’t hit a high number of IFFB like Posey. All hitters are different, but the preceding comparison is an example of why two hitters get different results on high and inside pitches. The infield fly ball is the worst batted ball result for a hitter since it almost always results in an out. Infield fly balls are usually induced by high-and-inside pitches that don’t have any downward breaking action. The only way to determine why a hitter has a propensity for hitting weak pop ups is to break down their mechanics. H/t to Mike Newman for helping with the swing evaluations. . . . . Appendix Count Count IFFB% 0–1 7.0% 2–1 7.1% 1–1 7.2% 1–2 7.3% 2–2 7.4% 0–2 7.5% 3–2 7.5% 3–1 7.6% 1–0 7.7% 0–0 7.8% 2–0 8.2% 3–0 9.1% The big surprise to me was the the higher number of pop ups on hitter counts. With hitters probably looking for a specific pitch at a specific part of the plate, the hitter bails out the pitcher by popping up the ball. Inning Inning IFFB% 1 7.3% 2 7.5% 3 6.9% 4 7.3% 5 7.3% 6 7.4% 7 7.5% 8 7.6% 9 8.2% 10 8.3% When looking at pop ups per innings, there is a drop from the beginning of the game through the next 3 innings. Then the rate slowly increases back up. Not much of a conclusion can be drawn of hitters from the data because the IFFB% increases the most when teams are bringing in their relief pitchers.

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