The Perils (and Perks) of Plus-Minus in the NBA

"You're sending my plus-minus through the roof! I love you, man!"

When you think of the best players in the NBA, a few names come to mind, like Kobe Bryant, LeBron James, and Dwight Howard. They score tons of points per game, and rack up assists and rebounds to boot. All three of them are All-Stars.

But when you hear names like Anderson Varejao, Al Horford, and Josh Smith, you don’t get the same image. Instead, you think of players that are talented, but not elite. Role players might be a good term for them. As impressive as he is, no one is confusing Josh Smith with LeBron James.

But look at these players through the lens of plus-minus, and everything changes.

Plus-minus is one of those terms you hear thrown around all the time, but probably don’t understand fully. Essentially, plus-minus is a measure of how well a player’s team performs while he is on the floor. If the Celtics outscore their opponent by 10 points while Paul Pierce is on the floor, then Pierce has a plus-minus of 10 for that game.

On the surface, that would seem to measure a player’s all-around contributions in a game, both on offense and defense. If Pierce is killing it on offense but routinely being burned by his man on defense, his plus-minus will be a lot lower. But, as you might expect, there are some mitigating factors here. Orlando’s Matt Barnes has one of the highest plus-minus ratings in the NBA, averaging about 4.6 per game, meaning the Magic outscore their opponents by about 4.6 points while he’s on the floor. Impressive numbers, to be sure, but anyone who watches Barnes has to wonder why a player averaging 8 points and 5 rebounds a game is rated so highly.

So what’s the deal? A quick look at Orlando’s lineups gives us our answer. Barnes plays virtually all of his minutes while Dwight Howard, a defensive menace, is on the floor. Not to downgrade Barnes’ contributions, but it’s not difficult to see your team outscore your opponent while the reigning Defensive Player of the Year is there with you.

Meanwhile, Anderson Varejao puts up very similar numbers to Barnes (8 points and 8 rebounds per game), but his plus-minus is good for fourth in the league, behind only James, Bryant, and Howard. Like Barnes, he too benefits from playing alongside a defensive presence in LeBron James. But Varejao also adds plenty of value through energy plays and defensive effort that doesn’t make its way into the box score.

So how can we weed out the pretenders and find the people that are really contributing? One way is to look at plus-minus ratings of pairs of players, showing which guys bring out the best in each other. For instance, when both James and Varejao are on the floor, the Cavs outscore their opponents by over 7 points on average. That’s a pretty healthy margin, and a sign that both are likely contributing (although it could be that only James is contributing, and that he and Varejao happen to play a lot of minutes together). Barnes and Howard make this list, too, with a combined plus-minus rating of about 5.2.

One interesting tidbit: of the top ten two-man combinations by plus-minus, the Hawks have a staggering six of them. Possibly something to remember when you’re placing bets on the postseason.

But if you’re still not convinced, there’s one more way to separate the boys from the men: adjusted plus-minus. This is a more complex statistical model that tracks a player’s plus-minus rating, but also takes into account the ability of the player’s four teammates on the court, as well as that of his five opponents. If the ’96 Bulls and I are playing the ’10 Nets, I’ll probably have a fantastic plus-minus for the game, since my teammates are so good and my opponents are so bad. Adjusted plus-minus helps take that into account and adjusts my score to reflect the fact that I’m 5’9”, completely out of shape, and a turnover machine.

As you can imagine, compiling all of that data is time-consuming, so those who take the time to put those stats together aren’t usually willing to distribute it for free. For most of us casual fans, simple plus-minus is sufficient. It gives us a good, general idea which players are contributing most (and best) in any given game. But as with any statistic, you have to take it with a grain of salt. In last night’s game between the Lakers and the Memphis Grizzlies, Kobe Bryant’s plus-minus was just 3, and was short of the 4 posted by Shannon Brown and Hamed Haddadi. Yet Bryant’s line of 32 points, 6 assists, and 7 rebounds almost doubled the combined output of Brown and Haddadi, and he made the game-winning shot, to boot.

So which player made the biggest impact on his team?

Statistics like plus-minus never tell the complete story, just like anything else. But they do help illuminate things we might not have seen otherwise, and that’s why they’re worth looking at.