ORtg on NBA.com vs Basketball-Reference (Team)

[Cruel_Ruin]

Anyone else notice the discrepency?

From Basketball-Reference:

Portland Trail Blazers-114.57
Miami Heat-112.05
San Antonio Spurs-110.68
Houston Rockets-110.18
Minnesota Timberwolves-109.69
Los Angeles Clippers-109.57
Dallas Mavericks-109.56
Oklahoma City Thunder-109.33
New Orleans Pelicans-108.62
Phoenix Suns-108.55

From NBA.com:

Portland Trail Blazers-110.30
Miami Heat-109.30
San Antonio Spurs-108.00
Houston Rockets-107.30
Los Angeles Clippers-106.50
Dallas Mavericks-106.40
Oklahoma City Thunder-106.40
New Orleans Pelicans-105.30
Minnesota Timberwolves-105.20
Phoenix Suns-105.10

Why are they different? They supposedly both measure points per 100 possessions.

[mysticbb]

Why are they different?

They are calculating the amount of possessions in a game differently, which means they get different numbers for pace, therefore different numbers for points per 100 possessions. When actually counting the possessions, basketball-reference is closer to the reality than nba.com.

[lorak]

Anyone else notice the discrepency?

From Basketball-Reference:

Portland Trail Blazers-114.57
Miami Heat-112.05
San Antonio Spurs-110.68
Houston Rockets-110.18
Minnesota Timberwolves-109.69
Los Angeles Clippers-109.57
Dallas Mavericks-109.56
Oklahoma City Thunder-109.33
New Orleans Pelicans-108.62
Phoenix Suns-108.55

From NBA.com:

Portland Trail Blazers-110.30
Miami Heat-109.30
San Antonio Spurs-108.00
Houston Rockets-107.30
Los Angeles Clippers-106.50
Dallas Mavericks-106.40
Oklahoma City Thunder-106.40
New Orleans Pelicans-105.30
Minnesota Timberwolves-105.20
Phoenix Suns-105.10

Why are they different? They supposedly both measure points per 100 possessions.

I'll use 2013 finals game 1 as example to explain it.

According to basketball-reference:
85.0 pace
SAS ortg 108.2
MIA ortg 103.5
so SAS +4.7 efficiency differential (and in reality Spurs have won by 4 points).

According to stats.nba.com:

87.7 pace
SAS 102.3 ortg
MIA 102.9 ortg
so SAS -0.6 efficiency differential

In reality (if we count possessions manually from play by play) Miami had 86 possessions and Spurs 84, so game pace was 85 and ortg/drtg exactly as basketball-reference says.

I contacted stats.nba.com and they said they are using simplified formula for possessions [FGA + .44(FTA) - OREB +TOV)] while basketball-reference more complicated, but that could only explain small differences in pace. The main reason why stats.nba.com numbers are way off (according to them Spurs lost game 1!) is because they calculate ortg/drtg not from game pace, but team pace. I mean, normally we calculate team A's and team B's possessions, add them and divide by 2 and that gives us game pace and then based on that we calculate ortg/drtg. That's what basketball-refernce does and it's very good method (error margin is very small if we compare estimated results to real - calculated manually from play by play). stats.nba.com on the other hand skips that "divide by 2" part and they calculate ortg/drtg from team possessions, not from game pace.

In short
basketball-reference: (team A possessions + team B possessions)/2 and then ortg/drtg
stats.nba.com: team A possessions and then team A ortg; team B possessions and then team B ortg