The Dream: Net Plus/Minus and Who You Want on the Court

There's a statistic that you'll find in WNBA box scores called "Plus/Minus". It is usually listed with a +/- symbol. I explained how this works on a messageboard, and I'll just cut and paste:

Assume that Betty Basketball (no relation to Betty Lennox) comes into a game two times.

The first time she comes in, the score is 20-20. When the coach finally substitutes for her, the score is 35-30 in favor of Betty's team. In effect, the team increased its lead by five when Betty was on the court. We give Betty a +5 for this interval.

The second time she comes in, her team is ahead 65-50. When she leaves, the score is 70-65 - clearly Betty's presence wasn't helping anything. The lead decreased by 10 points when Betty was out there. We give Betty a -10 for this interval.

These were the only two times Betty was on the court during the game: we add all of her plus-minuses together for each time she was on the court: +5 + (-10) = -5

Next to the box score, there will be a "-5" next to Betty's name. The implication is that the team did five points worth of worse when Betty was out there.

The whole point of +/- is to draw some sort of conclusion as to whether or not the presence of a particular player helps or hurts a team.

Of course, plus-minus isn't a perfect stat for a few reasons. The first is that plus/minus can depend greatly on who you're with on the court. If you're surrounded by All-Stars, your team will certainly do better when you take the court...but it might not be so much because of you. It might just be the fact that the other four players are picking the team up, and she's just along for the ride. Likewise, a player's plus-minus might be dragged down because she has crappy teammates.

The second problem is that the method above calculates something called raw plus/minus. Raw plus/minus is the first step, and everybody can do it. You just count points, and it requires no skill greater than addition or subtraction. However, raw plus/minus, being a cumulative statistic, rewards players proportionally to how much time they get on the court. The more time you spend on the court, the higher your numbers become (in either direction).

We can't fix the first problem, but we can fix the second one. We simply equalize the stats. In other words, we project all players to 40 minutes per game. This gives people who aren't on the court a lot a chance to shine.

WNBA Leaders in Net Plus/Minus
(values as of June 28, 2009)

1. Lindsey Harding, Mystics: + 48.3
2. Sue Bird, Storm: + 36.0
3. Barbara Farris, Shock: + 35.1
4. Lauren Jackson, Storm: + 31.2
5. Chelsea Newton, Monarchs: + 30.4
6. Candice Wiggins, Lynx: + 27.2
7. Candice Dupree, Sky: + 23.9
8. Jia Perkins, Sky: + 22.9
9. Seimone Augustus, Lynx: + 21.6
10. Katie Douglas, Fever: + 19.8

The numbers above pass the "smell test" - you would expect the players on the list to be in the top 10 in something, and the list isn't swamped with a bunch of no-name players.

Now, let's look at the Net Plus/Minus Leaders for the Atlanta Dream

Atlanta Dream Leaders in Net Plus/Minus
(values as of June 28, 2009)

1. Tamera Young, + 25.1
2. Sancho Lyttle, + 8.7
3. Iziane Castro Marques, + 5.7
4. Jennifer Lacy, + 3.0
5. Coco Miller, + 1.7
6. Shalee Lehning, - 0.1
7. Erika de Souza, - 2.0
8. Chamique Holdsclaw, - 5.0
9. Nikki Teasley, - 5.9
10. Michelle Snow, - 6.9
11. Angel McCoughtry, - 12.1

You might not expect Tamera Young at the top, but the net score indicates that the team performs better with Young on the floor than without her, despite Young's dismal shooting percentage. Are Little Smooth's teammates simply forced to pick up the slack when she's there, or does she bring something to the Atlanta Dream that can't be qualified in a box score?

Furthermore, Shalee Lehning suporters can take heart. Angel McCoughtry has only played about 11 more minutes than Shalee, but Angel's net plus/minus is significantly lower. Shalee, as a matter of fact, is right in the middle of the Dream in net plus/minus.

Initially, I wondered if individual style would have a lot to do with net plus/minus. Sue Bird is seen as a player who does the "little things" that don't show up on a box score but which help her team, whereas Angel McCoughtry is the "put-the-whole-team-on-my-back" kind of player - if Angel does poorly, so does the team.

However, the streaky Izi Castro Marques is #2 in net plus/minus for the Dream. Clearly, net plus/minus tells us more than we knew before - but we know not to rely too much on one metric.