Most studies of home field advantage (HFA) deal with team sports. This has both benefits and drawbacks. On the one hand, team sports normally involve teams with schedules balanced between home and road games, which makes it easy to measure the effect. On the other hand, the effect can only be measured by the outcome of games, which makes it difficult to find where the effect is coming from.
In a study called Home advantage in speed skating," econometrician Ruud H. Koning tries to isolate HFA for individual speed skaters. To do that, he directly measures the skaters' performance -- that is, their speed.
Unfortunately, though, you can't just measure speed directly -- you have to make all sorts of adjustments. For one thing, there are "park" adjustments, since skaters post faster times in indoor tracks than outdoor tracks. They perform differently at high altitudes than at low altitudes. Skaters as a group perform better over time, dropping a huge 7% off their times between 1987 and 2002. (Some of the improvement is caused by changes in equipment, such as something called a klap skate, which was introduced in 1996.) Average speeds are different for different lengths of race. Competitors, for some reason, perform better at major events such as the Olympics.
And so, Koning had to adjust all the observed speeds, by running a regression of log(speed) on all the above factors. His estimates of HFA were:
Men are 0.2% faster at home than on the road;
Women are 0.3% faster at home than on the road.
I hoped to compare these numbers to the HFA in team sports, like baseball. To do that, we'd want to know the answer to the question "if two exactly equal skaters faced each other, how often would the home skater beat the visiting skater?" Alas, the study doesn't give us enough information to tell. If we knew the standard deviation of results for an individual skater, that would be enough. But we don't have that. (For instance, suppose the SD was 0.14%. Then, the SD of the difference between two equal skaters would be root-2 times that, or 0.2%. The observed HFA would then be exactly 1 SD. The chance of a normal variable exceeding 1 SD is about 1/3, and so the road skater would be about .333 against the home skater.)
What Koning does tell us is that the average difference between Olympic gold and silver in 2002 was 0.77% for men and 0.45% for women. In baseball, the 162-game difference between home and road is about 13 games, while, typically, the best team in MLB beats the second best team by only a game or two.
So in skating, the distance between first and second is three times the HFA. In baseball, the difference between first and second is maybe 1/5 the HFA. This suggests that HFA has a much, much smaller effect in skating than in baseball.
I would have expected HFA in skating to be much higher. For one thing, there's a lot of luck in baseball -- the inferior team wins a reasonably large proportion of games. But in speed sports like skating, you'd think there's a lot less luck, and individual times would be pretty consistent. In that case, you'd think that any advantage in ability would translate fairly directly into advantage in observed performance.
Put another way: home teams play .540 baseball. But if you compared only strikeout rates, I bet home teams would play higher than .540. And if you compared ball/strike ratios, home teams would be better still. Speed skating seems more like a pure skill, more like throwing strikes than scoring more runs. And so you'd expect a larger effect.
Second, it seems like the regression result is so small that changes in the method might significantly affect it. For instance, the Olympic Games effect is four times the size of the HFA. If there are other confounding factors that weren't included, those could significantly change the size of the observed HFA.
In fairness, I don't know if there are any of those, and Koning seems to know a lot about skating and has added a fair number of variables based on his understanding. (And I think this study illustrates the importance of knowing about the sport you're analyzing -- researchers with little knowledge of skating, such as myself, might have remained ignorant of the sudden introduction of the klap skate, and used a linear variable for year instead of separate dummies to capture the abrupt change.)
But still: why is the Olympic effect is so large? Koning makes the argument that the Olympics feature competitors who are at the top of their game. But wouldn't they also be at the top of their game in the years immediately before and after the Olympics? That's an important variable that isn't included -- stage in a skater's career. Would adding that variable change the conclusions?
Or, less importantly, what if the klap skate improved short distance skating more than long distance skating? If you added interaction variables for distance and year, how much would home advantage change then? Probably not much at all, but maybe a lot.
But perhaps I'm nitpicking. Still, given that the effect is so small, and there are so many variables to adjust for, to my mind this study constitutes fairly weak evidence of HFA in skating.
Labels: home field advantage, skating