Tuesday, January 31, 2012

NHL Overtime Game Conspiracy - Update

Back in the first month of the NHL season, I noticed that games played between teams from different conferences had a much larger chance of going to overtime. It was still early days though, and the small sample size (94 games) had something to do with those surprising results (50% of non-conference games went to OT compared to only 15% of matches between conference foes). Now that we are at the all-star break and more than half the season is in the books, I thought it would be a good time to update the results.

Out of 735 total games played through January 25th, 164 (22.3%) have gone to extra time. Of the 554 conference tilts, 112 (20.2%) went beyond regulation while games featuring East vs West have an OT rate of 52/181 (28.7%). The numbers have obviously regressed towards the mean, which is what is bound to happen when you move from a small sample size to a larger one, particularly when the initial results are so skewed. Although there is a noticeable difference between the two, it is still not enough to cry foul. More data is required to prove something meaningful.

I looked at the results 2010-11 and found them to be quite similar. Including last season's 1230 games along with games so far this season, a more statistically significant result emerges:
       Total Intra Inter
Games  1965  1514   451
OT 461 334 127
% 23.5 22.1 28.2
The difference between 22.1 and 28.2 may not seem like a lot, but the it is actually 27.6% (that's 28.2/22.1). In other words, inter-conference games are 27.6% more likely to result in 3 points, a statistic worth considering when gambling in Las Vegas, if you could bet on this sort of thing.

Still, a season and a half is not much at all, so I continued to include prior seasons, planning to go back until 2005-06, which is when the shootout was instituted. However, from that season until 2007-08, clubs played only 10 non-conference games compared to 18 in later years. I decided to ignore these seasons as the difference in scheduling might bias the outcome. (As an aside, they didn't change the results much, mainly because there were so few non-conference games).

I was therefore left with 3.5 seasons to analyze. When I add 08-09 and 09-10 to the data, the theory vanishes, as you can see in the following table:
       Total Intra Inter
Games  4425  3434   991
OT 1044 798 246
% 23.6 23.2 24.8
So is the recent increase merely statistical noise or a changing trend? Let's look at the data by year. The following shows the percentage of overtime games by season.
Season  Total Intra Inter
11-12   22.3% 20.2% 28.7%
10-11 24.1% 23.1% 27.8%
09-10 24.5% 24.6% 24.1%
08-09 22.9% 23.8% 20.0%
As you can see, overtime is becoming more likely in non-conference games every season, while the other numbers bounce around a bit with no discernible pattern.

So do we have a conspiracy? Are teams playing to guarantee each the single point when East faces West? Doubtful. If they were, I think we'd see far more overtime games in these cases. As I said before, I think that clubs might change their late-game strategy when playing a close non-conference game, since allowing their opponent an extra point is not harmful in the conference playoff race.

Then again, we saw the opposite in 08-09. Naturally, such a limited data set makes any truly meaningful conclusion difficult; but the trend bears watching. I'll be paying close attention to the end of game action when I am watching the Leafs in Alberta next month, just to see if there is any noticeable change in strategy.




  1. Sean,

    What a beautiful statistical analysis of an interesting theory! I like it...brings back memories of all the fun we had in U. with all those stats courses.

    On a related topic, I see that the Sens are regressing back to their mean and losing a bunch of games. Still not too late to get in on the Vegas action though...look for more losses to come. I think that they were playing over their heads.

    Sean S.

    1. Thanks Sean. Watching the Senators now against the Islanders and hoping you are right tomorrow when they host the Leafs.