By Henry Johnson
The NBA playoffs have arrived at last.
With the first round series set, basketball analysts are looking to regular season results for information about postseason matchups. NESN, for instance, points to the Bulls and Celtics splitting their regular season series 2-2 as cause for concern. USA Today notes that the Clippers’ 3-1 record against the Jazz this season should inspire confidence among Los Angeles fans.
To test whether it’s wise to tie postseason predictions to regular season matchups, I grabbed data from Basketball-Reference going back to the 2006-2007 season. I was interested in home teams’ series winning percentage, as well as the following three regular season variables:
- Home team’s Adjusted Net Rating (ANR), an opponent-adjusted “estimate of point differential per 100 possessions,” as calculated by Basketball-Reference
- Away team’s ANR
- Regular season point differential between the home and away team.
As an example, this year’s Celtics have an ANR of +2.32 compared to the Bulls’ -0.08. The Celtics scored 409 points against the Bulls this season, while they allowed only 389. The regular season point differential, then, would be +20 for the home team Celtics.
A preliminary check suggests that home teams do fare worse against teams they’ve been outscored by in regular season matchups:
But we’ll need to control for team strength. After all, a negative differential may mean that, seeding be damned, the away team was actually better than the home team. To control for this, I used logistic regression to model the home team’s probability of winning each series as a function of the three variables outlined above: home team ANR, away team ANR, and regular season point differential in matchups between the two teams.
The results of this regression are as follows:
| Variable | Estimated Coefficient | Test Statistic | p-value |
| Intercept | 0.701 | 1.35 | 0.18 |
| Home Team ANR | 0.16 | 1.77 | 0.08 |
| Away Team ANR | -0.24 | -2.82 | 0.005 |
| Regular Season Point Differential | 0.02 | 2.04 | 0.04 |
These numbers indicate that regular season matchups do matter a significant amount, even after you’ve controlled for the strength of each team.
As an editorial aside, I should point out that I expected regular season matchups to be irrelevant, and I was fully prepared to point and laugh at the talking heads who suggest otherwise. Perhaps the professionals are professionals for a reason.
While the model is crude and simplistic, it might still be fun to predict the first round of the 2017 playoffs using our three variables of interest.
| Home Team | Probability | Away Team | Probability |
| Celtics | 82% | Bulls | 18% |
| Cavaliers | 89% | Pacers | 11% |
| Raptors | 86% | Bucks | 14% |
| Wizards | 95% | Hawks | 5% |
| Warriors | 91% | Trail Blazers | 9% |
| Spurs | 87% | Grizzlies | 13% |
| Rockets | 89% | Thunder | 11% |
| Clippers | 60% | Jazz | 40% |
These predictions should be taken with a grain of salt, especially compared to more advanced models. Whatever the outcomes, though, it’s good to return to the glorious time of year that is the NBA postseason.
Can you explain the “Home Team Winning Percentage” graph a little bit more. I know that is supplanted by your later analysis, but I could not fully understand what the graph is showing.
Is it showing the percentage of wins by team X during the season when playing at home against their first round playoff opponent binned by those with a positive/negative cumulative point differential for every game played in a season?
Thanks again for this really fascinating post!
Will
Hi Will,
Thanks so much for reading!
Sorry for the confusion on the graph. It’s showing percent of playoff series won when the home team had a negative point differential during the regular season against the away team (bar on the left) vs. percent for home teams whose regular season point differential was positive (bar on the right).
So the short of it is that outscoring your playoff opponents in the regular season is highly correlated with beating them in the playoffs.
Since the Celtics had a +20 point differential vs. the Bulls this year, they would be part of that group on the right.
Hope this makes sense!
Where on the Basketball Reference website can I find the ANR stats…..I am having a hard time finding the numbers.
Here’s the link to the current year’s ratings:
http://www.basketball-reference.com/leagues/NBA_2017_ratings.html
Thanks so much for reading!