Thursday, February 12, 2009

Coming off a Win/Loss: The effect of Prior Results

I’ve often wondered if the outcome of a team’s previous game has any affect on the result of that team’s subsequent game. Intuitively, I wouldn’t expect there to be much of an effect. The outcome of any given game is determined by many different factors, some of which are known to have a large effect.

While the result of the previous game could conceivably be one of these factors, it would probably rank pretty far down the list in terms of importance. In other words, if there is such an effect, I would expect it’s magnitude to be small.

That said, I’ve heard it argued before that the previous game does in fact have an effect on a team’s performance in the following game, so it’s something worth examining, I think.

On the one hand, some have suggested that the momentum of winning the previous game carries over to the next game, thus enhancing a team’s chance of success. According to this line of reasoning, the average team should do slightly better when coming off a win than when coming off a loss.

Conversely, others have suggested that winning breeds complacency, with losing having the opposite effect. This approach predicts that teams should do better when coming off a loss, on average.

I don’t think that either of these arguments have much merit. Both are based on the idea that psychological factors have a measurable effect on game outcomes, a premise with which I personally disagree. While casual fans often resort to folk psychology when discussing success and failure at the NHL level, its relevance has never, to my knowledge, been demonstrated through actual evidence.

In any event, I attempted to determine if the preceding game has any effect on following game results. My methodology was pretty straightforward. The sample included all regular season games played during the seasons of 2005-06, 2006-07 and 2007-08. Each game played was classified as a win, a loss, or a tie for both of the involved teams. For the sake of simplicity, any game that went past regulation was considered to be a tie. I then looked at whether that team won, lost or tied in its next game. Here are the results for 2007-08. The teams that had a better record when coming off a win compared to coming off a loss are shaded green. Teams for which the opposite was true are shaded orange.


Below is a chart of the average winning percentages of all 30 teams in each situation (coming off a win, coming off a loss, and coming off a tie) for all three seasons. The left hand column shows the average winning percentage for all 30 teams in games played after a win. The middle and right hand columns do the same, only for games where the team was coming off a loss and tie, respectively. It’s necessary to look at the average winning percentages rather than the aggregate winning percentages for one simple reason: better teams, by virtue of winning more games, tend to play a higher percentage of their games when coming off a win. For example, the Thrashers played a mere 18 games coming off a win last season; Detroit played 46. It needn’t be explained as to how this could confound the results.


Also included is a chart that breaks down the number of teams that had a better record after winning vis-à-vis their record after losing, and vice-versa.



The results are pretty consistent with my expectation in that the effect of the preceding game appears to be fairly small. In the 90 ‘team-seasons’ analyzed, 41 teams had a better record after winning, whereas the other 49 had a better record after losing. The average winning percentage for teams coming off a win was slightly less than 0.49. For teams coming off a loss, that figure was approximately 0.505. Therefore, it can be said that teams have, since the lockout, done slightly better after losing their previous game than they have when coming off a win. Of course, the margin is quite small and well within the potential range of random variance. Even supposing that the results are statistically significant, the influence of a team’s preceding game upon the outcome of its following game appears to be limited.

1 comment:

Cornelius Hardenbergh said...

Good stuff. You should consider cutting the images with the tables down so that they only have the tables and not the tables and a bunch of white space. Even the most basic image editors (mspaint, apple's Preview.app) will have the ability to do this.

Other than that, though, keep up the good work!