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Nikos
Joined: 16 Jan 2005
Posts: 346
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 Posted: Wed Jan 19, 2005 2:17 am Post subject: Expected Win-Loss differences? |
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| Why are B-R.com and BR.com's Expected Win%'s for a team reasonably different in results? Which is more accurate? |
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Ed Küpfer
Joined: 30 Dec 2004
Posts: 763
Location: Toronto
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| Posted: Wed Jan 19, 2005 3:17 am Post subject: Re: Expected Win-Loss differences? |
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| Nikos wrote: | | Why are B-R.com and BR.com's Expected Win%'s for a team reasonably different in results? Which is more accurate? |
B-R's is. But the differences are small:
Both esitmates use the Pythagorean Method. B-R uses an exponent of 14, giving a RMSE of 3.36. BR uses 16.5, with a RMSE of 3.81. To give you an idea of what those differences mean in less abstract terms, the 14 exponent gave an answer within 1 game of the actual win total 35.7% of the time, while the 16.5 exponent worked on only 32.2% of the time -- which works out, roughly, to 1 team more per season.
| Code: | Error Pyth14 Pyth16.5
+/- 1 game 35.7% 32.2%
+/- 2 games 56.5% 50.3%
RMSE 3.36 3.81 |
BTW if you're not familiar with the RMSE (Root Mean Squared Error), it is a meaure of the amount of variation of the errors. Roughly 2/3 of the errors will fall within 1 RMSE of zero in this case.
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Nikos
Joined: 16 Jan 2005
Posts: 346
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| Posted: Wed Jan 19, 2005 3:19 am Post subject: |
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Interesting, I noticed B-R.com has Expected W-L's to be 2-3 games less usually than BR.com's. I noticed it for the Spurs the past few seasons, they are always 2-3 games less.
And no I'm not familiar with your analysis below etc... I'll try and look into a bit, and learn  |
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Ed Küpfer
Joined: 30 Dec 2004
Posts: 763
Location: Toronto
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| Posted: Wed Jan 19, 2005 3:57 am Post subject: |
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I'll go into a bit more detail.
Both sites use the Pythagorean Method of estimating WIN%. It goes like this:
| Code: | WIN% = Points Scored^x
----------------------------------
Points Scored^x + Points Allowed^x |
where x an exponent, usually somewhere between 10 and 20. B-R.com has used 14 as their exponent, BR.com has used 16.5.
For example, last season the Pistons scored 7388 points, and allowed 6909. B-R would calculate their expected WIN% like this:
| Code: | WIN% = 7388^14 / (7388^14 + 6909^14)
= 71.9%
= 58.9 wins |
BR would do it like this:
| Code: | WIN% = 7388^16.5 / (7388^16.5 + 6909^16.5)
= 75.1%
= 61.6 wins |
Detroit actually won 54 games, so B-R was off by 4.9 games, and BR was off by 7.6.
If you look at the whole league, it looks like this:
| Code: | TEAM WINS B-R.com Error BR.com Error
ATL 28 27.5 -0.5 25.3 -2.2
BOS 36 36.7 0.7 35.9 -0.8
CHI 23 22.7 -0.3 20.0 -2.7
CLE 35 33.1 -1.9 31.7 -1.4
DAL 52 53.0 1.0 55.0 2.0
DEN 43 44.2 1.2 44.7 0.6
DET 54 58.9 4.9 61.6 2.7
GS 37 38.8 1.8 38.4 -0.4
HOU 45 46.6 1.6 47.5 1.0
IND 61 58.5 -2.5 61.1 2.6
LAC 28 27.9 -0.1 25.8 -2.1
LAK 56 52.3 -3.7 54.2 1.9
MEM 50 48.3 -1.7 49.5 1.3
MIA 42 42.7 0.7 43.0 0.3
MIL 41 44.1 3.1 44.7 0.6
MIN 58 57.2 -0.8 59.7 2.5
NJ 47 49.0 2.0 50.3 1.4
NO 41 40.7 -0.3 40.6 -0.1
NY 39 36.5 -2.5 35.6 -0.8
ORL 21 21.9 0.9 19.1 -2.8
PHI 33 33.1 0.1 31.7 -1.4
PHX 29 30.1 1.1 28.3 -1.8
POR 41 37.0 -4.0 36.3 -0.7
SAC 55 54.8 -0.2 57.0 2.2
SAN 57 62.3 5.3 65.2 2.9
SEA 37 39.1 2.1 38.8 -0.3
TRN 33 31.2 -1.8 29.6 -1.7
UTA 42 37.1 -4.9 36.4 -0.7
WAZ 25 24.8 -0.2 22.3 -2.5
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You can see that 12 of B-R's estimates (41%) fell within +/- 1 game of the team's actual win total. 11 (38%) of BR's estimates were within 1 game. 20 (69%) of B-R's estimates were within 2 games, while that number was 19 (66%) for BR.
2004 was actually one of the better seasons for Pythagorean predictions, both B-R's and BR's, but if you look through the history of the NBA, you can see a pattern repeated: B-R estimates are slightly more accurate than BR's. and both will predict the actual win total within 2 games of the true win total a little more than half the time.
Hope that helps.
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jkubatko
Joined: 05 Jan 2005
Posts: 702
Location: Columbus, OH
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| Posted: Wed Jan 19, 2005 12:35 pm Post subject: |
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Nice summary Ed. Thanks for saving me the time.
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Regards,
Justin Kubatko
Basketball-Reference.com |
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