Friday, 7 November 2014

Puckerings archive: Arena Goal Factors (29 Oct 2002)

What follows is a post from my old hockey analysis site puckerings.com (later hockeythink.com). It is reproduced here for posterity; bear in mind this writing is over a decade old and I may not even agree with it myself anymore. This post was originally published on October 29, 2002.


Arena Goal Factors
Copyright Iain Fyffe, 2002


One possible source of ideas when doing statistical analysis in hockey is analysis done in other sports. Of course, baseball is the most obvious choice, since so much statistical analysis has been done in that field. But one must be careful when importing ideas to consider the differences that exist between the sports involved.

The concept of park run factors is an example. Park runs factors exist in baseball because different parks have different dimensions and conditions, thereby affecting the number of runs scored in each park. Several people have suggested that such an analysis could be done in hockey, but to my knowledge, no one has published any results.

First, we must ask the question: do factors of this kind (let's call them Arena Goal Factors, or AGF) make sense for hockey? I would say yes, there could be enough differences between arenas (in terms of dimensions, ice condition, etc.) to affect goal-scoring levels. I would expect the differences to be less than in baseball, but would not be surprised if they do exist.

For example, let's take a team that scored 120 goals at home and 100 on the road, and allowed 90 goals at home and 100 on the road. In this league, teams score 55% of their goals at home, while allowing 45% of their goals at home. We would therefore expect this team to score 121 goals at home and allow 86 at home, for a total of 207 goals at home. They actually had 210 total goals at home. Their AGF is therefore 210 divided by 207, or 1.014.

But before we can use this figure, we have to adjust for the fact that a team plays only half its games at home, and half on the road (in other arenas with other AGF figures). Since the sum of league AGF is equal to the number of teams, we calculate the Arena Goal Adjustment (AGA) as follows:

AGA = [(TMS-1)x(AGF)+(TMS-AGF)]/[2x(TMS-1)]

Where TMS is the number of teams in the league. I won't bother with the derivation.
So if the team in the above example played in a 25-team league, its AGA would be 1.007, meaning that players on this team would have their scoring totals increased by about 0.7% due to playing in their particular arena.

That's the theory, anyway. But I won't string you along any more. You can calculate AGA's for each NHL team for each season, but they are not the result of the nature of the arenas. They are random chance.

I calculated AGA's for six NHL seasons: 1990/91, 1991/92, 1994/95, 1995/96, 1998/99 and 1999/2000. If AGA were meaningful, there would be a strong relationship between the AGA for a team one year and the AGA for that team the next year. The results of this inter-year correlation is as follows: between 1990/91 and 1991/92, 0.34; between 1994/95 and 1995/96, -0.05; between 1998/99 and 1999/00, -0.37. The average correlation coefficient is -0.03, which suggests the relationship is entirely random.

For further support, I calculated the correlations between goals-for factors and goals-against factors for each team. If the effects were real, then we would expect to see both goals for and goals against affected in the same way. The results of this intra-year correlation are as follows:

 Year  Correlation
 1990/91  0.13
 1991/92  0.24
 1994/95  0.05
 1995/96  0.24
 1998/99  0.32
 1999/00  -0.03

The average correlation is 0.16, which is stronger than the inter-year correlation, but still nowhere near as strong as we would need to say there is a relationship there.

In summary, Arena Goal Factors do not exist in hockey. You can calculate them all you like, but overall they are the result of random chance and do not reflect anything meaningful.

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