One of common themes you'll hear around here is, "Player Y has gotten lucky/unlucky so far this season and here's why..." I know when I first started reading here and at other statistically inclined sites, this concept struck me as totally bonkers. Luck in baseball? If a player isn't performing well, that's because he's in a slump. They're pressing too hard or they need to fix their mechanics...or something like that. Luck doesn't have any play in baseball - it's all about talent.
As it turns out, I wasn't asking that all important question (Why?) and doing the necessary research. So this player is in a slump - why is that? What can his numbers show us? You can tell if a batter is pressing by if he's changing his plate approach: swinging at more pitches, expanding his zone, striking out more, etc. You can also get an idea of if a player's mechanics are off by his batted ball types: is he hitting more groundballs or flyballs and less line drives? Obviously this isn't a perfect method, but unless you're a professional swing mechanic, that's the best we can do by looking at the statistics. If either of these things is off, you can begin to answer that question by saying, for example, "Player Y is slumping and look, he's swinging at more pitches outside of the zone and he's hitting a ton more groundballs. He needs to change his approach to get out of this."
But sometimes there's nothing to see in a player's batted ball profile or plate approach statistics. Sometimes a player looks like the exact same hitter that they were a month ago - still swinging at the same amount of pitches, still walking the same amount, still hitting the same number of line drives - and yet their batting average has plummeted by 50 points. What gives? If this is the case, you need to start thinking about luck.
Contrary to popular belief, luck play a large role in baseball. Primarily, this is because hitters can't control exactly where they hit the ball. That's not to say batters have no control over where the ball is going; we've all seen hitters purposefully hit sacrifice flies or hit a groundball to second base to move a runner over to third. To an extent, you can control where you hit a ball by the angle of your bat and your swing plane, and major league hitters are professionals that have an incredible amount of bat control. But then again, if batters could place a ball where they wanted every time at the plate, nobody would ever get out, right? Pitches are coming in at 90-ish miles per hour with spin and curve, and the difference between hitting a screaming line drive and a weak pop-up can be a couple millimeters one direction or the other. With such a small margin for error, batters simply can't say, "Oh, I want to put the ball directly in that hole at shortstop" and succeed every time doing it; that's a monumental task. And so, sometimes a player may make solid contact, but hit the ball directly at defensive players time and time again. Are they doing anything wrong? Have they changed their approach? No, it's just they've gotten unlucky.
And that's not even taking the quality of the defending team into account. Imagine that Evan Longoria hits a deep flyball to left centerfield. If it's Curtis Granderson out there - a fast, above-average defender - he may run it down and catch it. But if it's a player like Vernon Wells out there - a slower, below-average defender - he may miss the ball and Longoria would have a double. Oh, and what about the park he's playing in? If Longoria hit that exact same flyball in Fenway Park, with the short left field fence, he may have a homerun instead of a double. That's quite a range in possible outcomes - out, double, or homerun - from a single flyball, and Longoria can't do anything to influence those outcomes. He can only hit the ball and hope.
If you want to measure a player's "luck" on balls in play, there are a couple places you can look. The golden standard is Batting Average on Balls In Play (BABIP) and it's exactly what it sounds like: a player's batting average on balls that he hits in play. The league-average is around .300, but speedy players normally have higher BABIPs and slow hitters normally have lower. If you want to tell if a player has gotten lucky or unlucky recently, look at their career BABIP number and compare it to how they're performing at the moment. If their career number is higher, they've likely gotten unlucky to an extent; if their career number is lower, then the player has gotten lucky.
Another place to look for luck is a player's homerun per flyball rate (HR/FB). Power hitters normally hit homeruns on around 13-20% of their flyballs, while contact hitters typically have a HR/FB rate in the single digits. If you see that a player like Reid Brignac is hitting homeruns on 15% of their flyballs, you can probably safely assume that his power surge is not going to continue. He may or not be getting "lucky" per se - it may just be an incredible hot streak - but it's not how he's likely to perform in the future.
And so, in the future, if you see a player that's slumping or on a hot streak, ask yourself "Why?" Dig into their statistics and try to determine if the player has changed their approach, or if they've had Lady Luck mess with their balls in play. Sometimes it's easy to tell - the player has a high BABIP and nothing else in their statistics screams "Anomaly!" Sometimes, though, the answer is a little of both - a player may have a high BABIP, but they're also hitting more line drives (the batted ball type that's most likely to fall for a hit). That player still will likely cool off once they stop hitting so many line drives, but what if they've made an adjustment to their swing and so they'll keep hitting an increased number of line drives? Maybe they'll cool off a little bit, but not as much as you'd first thought. Everything is interconnected and meaningful, so be sure to take a comprehensive look at a player's statistics before you scream "Luck!" or "This guy sucks!"
For more information on BABIP, homerun rates, and luck, please refer to The Sabermetric Library or ask questions below.