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What Do We Mean By Luck? An Explainer

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As of Friday, Vanderbilt's basketball team ranks 341st (out of 351 teams) in "luck," according to Ken Pomeroy. But just what does that mean? Is Vanderbilt really just unlucky, or is something else going on?

Phantom fouls: an example of bad luck.
Phantom fouls: an example of bad luck.
Spruce Derden-USA TODAY Sports

In sports, "luck" is a loaded term.

Many sports fans don't like to admit that sheer good or bad luck can affect the outcome of a game, or of a season.  That's particularly true of fans of teams that have benefited from good luck -- don't you dare suggest that their team isn't all that good and has just been lucky.  But it also seems to be true of teams that are the victims of bad luck.  When a team plays six games that go down to the wire and loses all six of them, some fans want to treat that as an indictment of the team for their inability to win close games, never admitting that such a string could just be a matter of luck.

Currently, Ken Pomeroy has Vanderbilt ranked #341 in the country in luck.  There are 351 teams in Division I.  So, according to Pomeroy, the Commodores have been quite unlucky.  But just what does that mean?

In Pomeroy's parlance "luck" simply means the deviation from expected winning percentage based on the team's overall performance and schedule.  Vanderbilt's Luck rating is -.107, or about 2.5 games (out of 24.)  So, Pomeroy is saying that where Vanderbilt actually has a 14-10 record, they should really be more like 16-8 or 17-7 with average luck.  Conversely, conference foe South Carolina ranks #13 in Luck; with average luck, they would be 18-6 or 19-5 rather than the 21-3 record they actually sport.

So, one general meaning of "luck" refers to the timing of positive and negative deviations from average performance.  Few teams play at the same level in every single game, and the timing of good and bad performances at the margins can affect a team's overall record.  If a team's worst performances all happen to come in 50-50 games, and their best performances come in 90-10 games (games they are highly likely to win, or conversely highly likely to lose), their record will likely be worse than it would be otherwise.

You can see some reflection of this in Vanderbilt's record.  The Commodores' best performances, statistically, have tended to come against teams like Austin Peay, Gardner-Webb, and Detroit -- teams that Vanderbilt was already extremely likely to beat.  A great performance against Detroit just means Vanderbilt won by 50 instead of 30.  On the other hand, a relatively weak performance at Ole Miss means the Commodores lose by 7 instead of potentially winning the game, as they were favored to do.  If you reverse those performances -- Vanderbilt beats Detroit by 30, and beats Ole Miss by 13 -- the record is better, but statistically it's the same team.

Now, this isn't 100 percent true, as Vanderbilt has had some great performances against good teams -- see: Texas A&M -- as well as some uninspiring performances against bad teams (specifically, home wins against Auburn and Missouri.)  But this is more or less the Ken Pomeroy definition of "luck."

What else can we chalk up to luck, though?  In another sense, luck refers to anything outside of a team's control that nevertheless affects the outcome of the game.  Things like:

-Officiating. Duh.  Officiating is the single biggest factor that the teams have no control over, and which can affect the outcome of a game.

Now, this doesn't necessarily mean bad calls or biased officiating.  Sometimes, it can affect the game when officials are calling the game "tight" or "loose."  When the refs are calling a foul on pretty much every possession, this rewards teams that are good at free throw shooting, and it also rewards teams that have a lot of depth (because of the high likelihood of players accumulating fouls and going to the bench or, especially, fouling out.)  When the refs are calling the game loose, letting a lot of contact slide and calling relatively few fouls, this works to the benefit of physical teams, which can alter shots near the basket simply by making contact with the shooter.

-Opponents' shooting. This is a tricky one.  A team's own shooting percentages are not a matter of luck, but the other team's shooting percentages can be.

Most clearly, a team has no control over its opponents' free throw shooting.  You can exercise some control over how many free throw attempts the other team gets (though, see above about officiating; you can control whether you're committing fouls, but not whether the fouls are actually called), but once the other team's shooter is at the line, you have no control over whether he makes or misses the attempt.  So, when a 70 percent foul shooter gets to the line ten times and makes four, the defense has gotten lucky -- that's three points the other team probably should have that they don't.  When a 70 percent shooter goes 10 for 10, that's bad luck.  Normally, he makes 7, so that's three points the other team has that they probably shouldn't.

But even on shots from the field, luck comes into play to a certain degree.  This shows some of the interplay between luck and skill: the defense does exercise some control over the quality of shots available to the offense, but the defense has no control over whether those shots are taken, and once the shot goes up (unless it's blocked) the defense has no control over whether it goes in.

You can pretty clearly see the effects of luck when the offense misses an uncontested layup (good luck for the defense) or makes a halfcourt shot (extremely bad luck for the defense.)  In the former case, the fact that an uncontested layup was available means that the defense did not do its job; the fact that the offense came away with no points means the defense got lucky.  Likewise, few would argue that the defense should be spending much time preventing the offense from chucking up shots from the half court line, but occasionally one of those shots will go in.

But even at the margins, luck can come into play.  When a 20-percent three-point shooter gets the ball at the three-point line, many times a team will simply lay off and allow him to take that shot.  Of course, sometimes the shooter decides (wisely) not to take that shot even though the defense is daring him to take it.  But sometimes the defense is lucky and a poor shooter does start taking those shots.  If a 20-percent three-point shooter attempts 10 three-pointers, that's good luck for the defense (as the offense is intentionally taking bad shots.)  But sometimes, that guy will randomly hit half of his shots in a given game.  That's bad luck.  And sometimes, a great shooter will just randomly have an off night shooting the ball.

Now again, your own shooting percentages are not a matter of luck.  But the other team's shooting percentages can be: sometimes the defense is suffering breakdowns left and right, and yet the offense keeps missing shots; and sometimes, the offense is making circus shots that really shouldn't be going in very often.

-Unforced errors by the other team. Now, again, your own unforced errors are not a matter of luck.  But when the other team is committing boneheaded mistakes, over and over again, that's luck.  Sometimes an active defense is forcing a lot of turnovers (maybe a hint of luck there, as the offense could still not commit turnovers despite the defense's best effort), and then sometimes an offensive player just airmails a pass into the second row of the stands.  Sometimes, an offensive player dribbles the ball off his foot.  It happens.  These kinds of turnovers are gifts -- the defense isn't really doing anything except watching the other team implode.

Aside from turnovers, the other team can commit other unforced errors.  Sometimes they call a timeout they don't actually have.  Sometimes, a player on the other team drops the f-bomb in front of a ref and gets a technical foul.  Sometimes, the opposing coach just randomly decides to bench his starters and play a bunch of walk-ons because he doesn't like their effort in practice.  You have no control over the opposing team doing dumb stuff, but it does work to your advantage when they do.

Now, in games that are decided by a large margin, luck doesn't come into play.  If you're losing by 20 points, it isn't luck that's causing you to lose.  But in close games, simple luck can mean the difference between a win and a loss.

So, is it just bad luck that's causing Vanderbilt to have an underachieving record, as Ken Pomeroy says?  Or is it something else?  It's really hard to say given all the evidence both ways that we've seen this season.