The "Differential" Indicator: First Quarter-Season Report (Part I)
During the early rounds of the playoffs last season, I performed a statistical survey and then posted the results of my search for a single statistical metric (using already published data from a team's regular season performance) that could separate out the NBA's True Contenders for the championship. So I looked back at 32 seasons of NBA data concerning team offensive efficiency (OE) and defensive efficiency (DE) rating differentials (i.e., OE rating minus DE rating, using basketball-reference.com). The hypothesis was that the teams that combined offenses that scored efficiently with defenses that forced their opponents to score less efficiently were the teams with the best chances at the ring.
As it turned out, this select group of true contenders consisted of eight out of the thirty teams (just one outlier team finished below 8th in differential and still went on to win the title - the 1994-95 Rockets, who finished 9th).
The following are just some of the key findings from that 32-year statistical survey:
- The median differential for title winners over the last 32 years is 6.8 (the 2011-12 Spurs differential was 7.1).
- Excluding one outlier at each end, the range in differentials for title winners is between 3.6 and 12.2.
- 59.4% of all title winners were ranked in the top two in differential.
- 75% of all title winners were ranked in the top three in differential.
- 81.3% of all title winners were ranked in the top four in differential.
Since finishing in the top four in differential (OE-DE) accounted for over 80% of the eventual title winners over the past 32 years (last year made it 33 years, since the Heat ranked 4th), using this formula does seem to give a rather accurate view of a particular team's relative odds of winning a championship. Thus, I would feel more comfortable if the Spurs finish in the top half of "true contender" teams (4th or better in differential) compared to a much less auspicious 8th place ranking (which barely keeps a team in the "true contender" group), since only 9% of teams over the past 33 years have won the title finishing either 8th or 9th in differential and absolutely no team below 9th has brought home a title.
Based upon this background, there does appear to be some validity in using differential as sort of a crude predictor of title chances. This post will be the first installment of four between now and the end of the regular season (about every 20th Spurs game) that looks at the evolving nature of differential as the season progresses. It will give us the opportunity to look at certain trends that could conceivably provide some sort of predictive value earlier than the end of the regular season.
The Differential Rankings
Also, I have made a minor methodological change in terms of data usage in the interest of ease of access to team offensive and defensive efficiency numbers and their corresponding differentials. I've used John Hollinger's team offensive efficiency and defensive efficiency data (instead of basketball-reference.com "ratings"), because that data is tabulated in a way that makes computing differentials a bit easier. Also, both sets of data measure essentially the same thing and appear to be highly correlated with each other. Offensive/Defensive "ratings" were used in the initial survey because these data has been collected over a much longer period of time and provided a reasonable sample size to establish important baselines and thresholds using the "differential" metric (e.g. no team has won a title in the past 33 years ranked below 9th in differential) and other factors (e.g., no team has won a title with a regular season win % of below .570, etc.).
The following table shows season-to-date offensive efficiency scores (OE), defensive efficiency scores (DE), team efficiency differential scores (EDS), and rankings (DR) for the top eight teams from both the Eastern and Western Conferences. In addition, the table gives the number of games played (GP) by that team to date, each team's win/loss record (W/L), their strength of schedule (SOS) numbers, which are simply the winning percentages for a team's opponents, and Hollinger's Power Rankings (PR). Eventually SOS statistics may allow one to effectively predict true contender status at some point before the end of the regular season by establishing certain thresholds, while still preserving its predictive value (it would be helpful though if SOS were somehow adjusted to take into account conference strength). Tracking SOS allows you to look back and provide reasonable explanations for why a team appeared to be a contender at Game 20, but failed to be so at Game 82.
|DR||Team|| Conference || GP || W/L || EDS || OE || DE || SOS* || PR
* Keep in mind, SOS numbers are not apples to apples for both WC and EC teams. These numbers should generally be adjusted higher by some unknown factor for WC teams (or vice versa lower for EC teams), mainly because the East simply has weaker teams on average. It would be nice if Hollinger made this statistical adjustment in his power rankings, but he does not as far as I know.
Contenders or Pretenders?
Only 20-25% of the season has been played by these eight teams thus far. Nevertheless, the Atlanta Hawks are probably the only surprise in the top eight (some might disagree, particularly in light of its most recent win in Memphis). This observation seems to be borne out by the fact that they have the lowest differential score, play in the weaker conference, have the weakest SOS, and have played the fewest games to date, all of which tend to weaken the validity of their ranking at this early juncture. However, barring unforeseen circumstances, I'm pretty confident the other seven teams will remain in the top eight at the close of the regular season. It is also highly likely that at least one of the following eight teams will or should ultimately crack into the top eight and kick the Hawks out. But the big question is which one will ultimately win that battle?
Chicago? (DR = 9th)
Brooklyn? (DR = 10th)
Minnesota? (DR = 11th)
Golden State ? (DR = 12th)
Boston? (DR = 13th)
Denver? (DR = 14th)
Indiana? (DR = 15th)
Philadelphia? (DR = 16th)
All of these teams have talent and the ability to make a move, but this first quarter report suggests that they are not currently meeting expectations and have some work to do to get into "true contender" territory. Certainly some of the younger and less experienced teams on this list, such as the T-Wolves & Warriors, would have to be considered long shots to make it into the top eight, but cannot be eliminated entirely at this early juncture. Reasonable excuses abound for all of these teams, such as player integration and injury issues, but do any of them have the ability over the next 60 games to leapfrog into the top eight? And if the Hawks are not the only team destined to make an exit from the top, who among the other seven teams is a dark horse candidate that could also conceivably be replaced?
Some might question how a snake-bit, sub-.500 and much maligned Lakers team managed to even make it into this exclusive group at the end of the first quarter. Could the vaunted, hyped-up future hall-of-famers in purple and gold be that dark horse? Well at present, they are the only team in the top eight with a win percentage less than .682, checking in at a paltry .450. And if they still have title aspirations this season, their win/loss record better jump considerably in the coming months, because no team has ever won a title with a win percentage of less than .570. And not only is their record poor, but they've had the 3rd weakest schedule and have played 12 of their first 20 games at home (60%). All of these facts certainly indicate a less-than rip-roaring start for this star-studded team.
Editor's Note: Part II, an analysis of the Western Conference contenders, will be posted tomorrow.