by Jim Glass
obsession, er, interest in the role that luck plays in close games got a sudden endorphin boost a couple weeks ago when an "OT game" button appeared on PFR.com's Game Finder utility. Data on all the sudden death OT games ever played was sitting just mouse clicks away. Joy!
No game is closer than a tie going into sudden-death overtime, so how do strong teams perform against weak teams in OT games?
1) Take all the overtime games played during the last ten years, 2000-2009, 155 of them.
2) For each team in each game, note its Simple Rating System point number as per PFR.com. This is just the team's average "points for" minus "points against" difference per game, adjusted by strength of schedule.
3) Use "Pythagorean expectation" to determine the expected outcome of full games between the contesting teams as indicated by each team's SRS rating, and compare it to the actual outcomes between them in OT games.
(Pythagorean expectation is a method of using points for/against ratio to estimate probable future won-lost record. First devised by Bill James for baseball, it is now widely applied to other sports. It is notably more accurate than past W-L performance at predicting future W-L performance.)
The idea that the outcome of overtime games is highly influenced by luck is hardly new. The goal here is to quantify by how much it is so. By comparing the results of teams of different strengths in overtime, and comparing them to the results of the same teams in regular games, in principle it is possible to estimate how much the results produced in overtime reflect team strength and luck respectively.
For instance, if the Pythagorean projection winning percentage for the stronger team in regular games is 70%, the random chance percentage is 50%, and the actual winning percentage in OT is 55%, we might conclude that OT outcomes are 75% chance and 25% the result of the better team's superiority.
In the 155 overtime games...
* The stronger team won 76 (49%).
* The weaker team won 79 (51%).
Well, that pretty much seems like 'nuff said. Everything else can you can deduce for yourself from that. You can stop reading now, if you want.
But since I'd set up a spreadsheet to produce a bunch more of parsed numbers, I'll give them for any who are interested.
In all 155 games total, the stronger team was on average 5.8 points better than the weaker. The Pythagorean projection for this strength difference is 66% wins for the stronger team, 102-53. The actual result of 76-79 was 49%.
Here are the results by relative strength of the contesting teams:
[Given are the favorite by SRS points (stronger team minus weaker), W-L, the average SRS point advantage for the stronger teams in the group, and the Pythagorean expectation win percentage for that point differential.]
favored by : W-L : win % : average SRS difference : Pyth expectation
0 – 2.9 pts: 22-33, .400, 1.49 pts, 54%
3 – 5.9 pts: 18-15, .545, 4.27 pts, 62%
6 – 8.9 pts: 17-17, .500, 7.49 pts, 70%
9 – 11.9 pts: 8-10, .444, 10.57 pts, 77%
12 – 14.9 pts: 5-1, .833, 13.10 pts, 82%
15 – 17.9 pts: 5-2, .714, 15.70 pts, 86%
18+ pts: 1-1, .500, 18.5 pts, 90%
TOTAL: 76-79, .490, 5.8 pts, 66%
Teams that were better by up to 12 points per game in full-length regular season games went only 65-75, 46%, against their weaker opponents in overtime.
These results for the last ten years provide no support whatsoever for the idea that the best teams win overtime games. If we accept that there's no reason for the better teams to lose systematically in overtime, the evidence here is consistent with the idea that these games are determined entirely by random chance, the equivalent of coin flips.
At an over-12 point differential the 11-4 record of the better teams might support the notion that teams that are much better than their opponents have a better chance to win in overtime – but that's a very small sample, not inconsistent with the coin-flip idea either. (For the record, an over 12-pt edge is an over-80% Pythagorean win expectation, and only about 10% of OT games are in that category.)
HOME FIELD ADVANTAGE
Originally I didn't bother looking at home field advantage because...
(1) Whether the home team is the stronger or weaker team seemed likely to balance out by chance, as it pretty much does, and so shouldn't much affect the numbers above, and
(2) Research at this site and elsewhere shows that home field advantage is strongest at the start of the game and fades later in the game – and you can't get later than overtime.
But what the heck. Of the 155 games the home team won 86, or 55%. That's the equivalent of being better by 1.8 points, as per Pythagorean expectation. In addition, as it happened, the home teams as a group were weaker by an average 1.2 SRS points than the visiting teams. So the home teams outperformed expectations by the equivalent of 3 points -- which often is cited as the number for "home field advantage".
Make of that what you will. I find it odd that home field advantage would reassert itself at the very end of the game in overtime while the real strength difference between teams disappears, but perhaps it is so. Or perhaps as 155 isn't a huge sample size this 55% number is just random variation.
I'm rather surprised to see zero reflection of team strength in overtime results (at least up to 12 points of differential).
It's logical to think that while chance factors may have a majority effect in determining overtime game results, yet still, the stronger team should score more frequently than the weaker team – and the team that scores more frequently should have an edge in scoring first to win in sudden death. Moreover, this logic is backed up in data from other sports. In baseball's semi-sudden death extra-inning games outcomes are determined mostly by chance, but the better team does have a slight edge which increases as the gap in quality level between teams grows.
Possibly this is the true state of affairs in NFL football too, but the slight advantage of the better team is invisible because of the curse of the NFL statistical analysis: small sample size. In baseball there are many thousands of extra-inning games to examine, in the NFL there are only about 15 OT games a year. So it's not possible to get more than a rough picture.
Anyhow, as a practical matter, as there is no sign of the better team having any edge in winning in overtime that's visible at all in a full decade's worth of games, one can conclude that overtime games between teams of different strength are effectively determined entirely by chance.
As to home field advantage helping a team win in OT, I'm agnostic, you can make up your own mind. But I wouldn't count on it.
For the larger picture I take this as more evidence that close games are determined overwhelmingly by luck. And, thus, that the formula for a championship team is to win games by a lot, not lose games by a lot, and be lucky enough in the close ones. Teams should be built and coached to play to dominate from the start, taking whatever calculated risks are necessary. The alternative strategy of playing safe to stay close and "have a chance to win at the end" with superior execution is a loser. It sacrifices chances to win big and leaves the team depending on luck in close games anyhow – only now in more of them.
Saturday, December 11, 2010
by Jim Glass