I Beg To Differential

Rob L. —  January 24, 2007

Rankings, Rankings, Rankings The NBA is far enough into the season that rankings such as Stein, Hollinger, Sagarin and FOTS (Friend of the Site) Knickerblogger generate heated debate. We the fans often go with our gut in response to outsiders ranking our team.

“Come on, Hollinger! The Lakers aren’t even in the top ten. They’re totally a top ten team.”

As we yell at the top of our lungs, the Staterati attempt to explain the why and how of their systems. There are real differences among the rankings, but one thing you’ll hear just about everyone mention is point differential.

A Win Is A Win, Right? The concept is simple. How much has a team outscored its’ opponents by? This idea migrated to basketball via Bill James. James found a strong correlation between runs scored/allowed and winning percentage in baseball. You can find the formula James used, as well as Dean Oliver’s basketball version here. Because this formula predicts team success fairly accurately, many use it to state what a team’s expected win % is.

And the short hand version of that is, “What’s that team’s point differential?”

Best In The West Which brings us to the debate raging over which team is currently tops in the Western Conference. The three teams in contention are the Mavs, Suns and Spurs. For whatever reason, most people (myself included) think of the Spurs as a little too old. This leaves the Mavs and Suns. Some go with record and pick the Mavs. But there are those out there that look at the point differential and pick the Suns.

All statistics compiled through Monday January 22, 2007

Mavs 34-8, +7.1
Suns 32-8, +8.9

The Mavs had played two more games than the Suns, but their records were comparable. Yet the Suns, on average, beat their opponents by 8.9 points per game. The Mavs, only by 7.1. In fact, the Suns sported the the biggest point differential in the NBA. And that’s just points per game. If we break it down into offensive and defensive ratings (points per 100 possessions) the Suns sport a +10.4 differential and the Mavs +7.7. The Suns are even further out ahead.

Here’s why I think that doesn’t tell the real story in this case.

Strength of Schedule Phoenix has had a much weaker schedule than Dallas to this point. Here’s how it breaks down.

Mavs vs Top 10 Teams 12-6, vs Top 16 18-6
Suns vs Top 10 Teams 4-7, vs Top 16 12-7

Sagarin’s Top 10 Teams are, in order: Dallas, Phoenix, San Antonio, Utah, Houston, LA Lakers, Chicago, Washington, Denver and Detroit. The Top 16 include: Cleveland, Orlando, Minnesota, Indiana, Toronto and New Jersey. If you look only at win-loss record, everything would stay the same except that the Bulls and Cavs change places in the 10/16 breakdown.

The first thing you’ll notice is that Dallas has played 18 games out of 42 against Top 10 teams. 24 out of 42 against Top 16 teams. That’s slightly more than half against the top half of the NBA. Phoenix has played only 11 games out of 40 against the Top 10. They played 19 of 40 against the Top 16, which is much more comparable to Dallas’ schedule. But it is clear that Dallas has faced more elite competition. My thought then, is that the Suns point differential is in part so high because they have beaten up on lowly teams.

The Test What I did was look at the Mavs first 42 games and the Suns first 40. (I know, the sample isn’t identical, but I’m just looking for close estimates.) I compiled point differentials for both teams against Top 10 teams and Top 16 teams. I further compiled two sets of Top 10 numbers: One where the Bulls were in it, the other had the Cavs instead. My hope was to see how the two teams played against similar opponents. Here’s what I found:

Mavs Top 10(Bulls) +2.11, Top 10(Cavs) +1.53, Top 16 +3.83 points per game
Suns Top 10(Bulls) -0.27, Top 10(Cavs) +1.36, Top 16 +5 point per game

And if we switch to offensive rating:

Mavs Top 10(Bulls) +2.39, Top 10 (Cavs) +1.73, Top 16 +4.33
Suns Top 10(Bulls) -0.29, Top 10(Cavs) +1.43, Top 16 +5.25

In both examples the Suns still hold the edge in point differential against the Top 16. But if we look at the Top 10 the Mavs hold the advantage. So at the moment, I’d have to say they are the better team. After Phoenix plays some more Top 10 teams I might come to a different conclusion. But I reserve judgment for now.

[Neither team has that many outlier games. The Suns had one win by 19 and another by 30. But most were closer to ten or less. The same is true of the Mavs who had wins of 27, 19 and 19 along with offsetting losses of 31 and 22 (the first was early season).]

Wrap It Up, Man! Point differential makes sense. It is a useful tool that has proven itself over time. But we must be careful not to use it without thinking. The current Mavs-Suns case is an example of how other factors can corrupt point differential.

Oh yeah. The Lakers are 9-7 against the Top 10, 17-7 against the Top 16. But their point differential is only…

“Whatever. The Lake Show’s going all the way, baby!

-Rob

Rob L.

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