NHL teams vary more in defense than offense
In the NHL, the spread of goals allowed is wider than the spread of goals scored. For instance, in 2010-11, the standard deviation of team goals was 22.0, but, for team goals allowed, it was 25.9. That is: there's more variation in defense than offense.
It's not the same in other sports. In baseball, since 1971, offense and defense have been almost equal -- a 102.4 run SD for runs scored, and 103.7 for runs allowed. For the NBA, it goes the other way -- offenses vary more than defenses. In 1978-79, the SD of team points per 100 possessions was 3.26, but only 2.64 for opposition points. In 2006-07, it was 3.78 to 2.64. (Those were the only two years I checked).
(In the links above, you have to do your own SD calculations. I wish the Sports-Reference sites listed SDs along with averages, to save time for geeks like me.)
Averaging out those two NBA seasons, and the NHL from 1995-96 to 2011-12, and switching to a nicer font, and making it bold to catch your eye in case you're just skimming:
NHL: 24.8 offense, 28.7 defense
MLB: 102.4 offense, 103.7 defense
NBA: 3.5 offense, 2.6 defense
It might seem like offense and defense should be equal, since one team's offense is the other team's defense. But that's not necessarily true. Often, it's easier to influence your score than your opponent's score. Like in golf. Players' scores vary quite a bit; Tiger Woods averages low, other guys average high. But *opponents'* scores would be almost the same for every golfer.
That, of course, is because there's actually no defense in golf -- you don't have any control over your opponents' scores, so there's no intrinsic variation in how well Tiger Woods scores against different opponents. That's an extreme case ... but it's easy to imagine "partial" cases. Imagine if MLB decided to use pitching machines instead of actual humans. Now, you can no longer keep your opponent off the scoreboard with good pitching. You only get fielding, which gives a much narrower range. Gold Gloves will still help, but the difference between the team allowing the most and fewest runs will be much narrower.
For the other way around, imagine MLB using a batting machine for the designated hitter. Now, your team's hitting skill matters only 8/9 as much as it used to, and it's a bit harder to influence your own scoring, relative to the opponents'.
So, the balance between offense and defense depends on the structure of the game. In fact, I think the near-equal balance in baseball is in large part just coincidence.
Another factor is how much one or a few exceptional players can influence the results. The more players you average out, the lower the SD. It's like how the distribution of heights among individuals is more extreme than the distribution of average heights among groups of individuals. It's easy to find someone who's 6-foot-4 ... but it's nearly impossible to find a neighborhood, or even a street, where the average is 6-foot-4.
Imagine changing baseball so that you only have one batter instead of nine. (If the one batter gets a hit, a pinch runner comes in, so the same guy can bat again.) In that case, team offense would have a much wider range than team defense. The team with Barry Bonds would score, say, 12 runs per game, while the team with Albert Pujols would wind up with only 7 or 8. That's a much larger range than real life. On the other hand, the range of defense would be narrower, since you can't have Pedro Martinez pitching every at-bat, and not every ball can be hit to Ozzie Smith.
So, here's an idea. When you look at a sport, search for the tasks at which there's wide variation in player talent, and where those tasks can be concentrated the most on certain players. If those tasks tend to impact scoring -- like in "one player bats" baseball -- the offense will have the wider spread. But if those tasks tend to impact more on *preventing* scoring, it's defense that will vary more.
In the NBA, where do you find that kind of task? Shooting is the most obvious -- there's wide variation in skill, and you can easily arrange to give your best players the ball more often. So, you'd expect wider team variation in offense than defense. Of course, the guy who shadows Kobe gets "concentration" on the defensive side, because he gets the most important job most often. But, intuitively, there's probably less variation in ability to defend good shooters than there is in ability to shoot against good defenders. (Of course, I say that knowing the result in advance, so that may just be benefit of hindsight. But I still think it's right.)
What about baseball? You don't have a lot of concentration on offense; the better hitters, at the top of the lineup, bat a bit more often ... but that's about it. On defense, your best pitchers get a few more innings. It seems kind of even. And, hitting and pitching seem about equal in importance (again with the benefit of hindsight). So you get roughly an even spread.
What about hockey? Where do you find a wide varation in talent, concentrated in just a few players?
You've got five guys who work together on offense, but *six* guys who work together on defense -- and the sixth one has the most important position, and he's almost always the same guy, getting maybe 80 percent of the ice time. So I think that's why, in the NHL, defense has a wider spread than offense -- because of the goaltender.
The numbers support that, kind of.
A while ago, I ran some estimates for the talent distribution of number-one goaltenders. I got that the SD of talent, in terms of shooting percentage, was around .008. With around 1700 shots per season, that's 13.6 goals.
It turns out, that's almost exactly makes up the difference between the spreads of offense and defense! The SD of goals allowed 28.7. The SD of goaltending talent was 13.6. If you subtract the square of 13.6 from the square of 28.7, you get ... the square of 25.3.
That 25.3 is very close to the SD of goals scored, which was 24.8. You could credibly argue that if every team had the same caliber goalie, the spread in team offense would be the same as the spread in team defense.
13.6 -- SD of goals allowed attributed to starting goalie
25.2 -- SD of goals allowed attributed to the other players
24.8 -- SD of goals scored
But, as I said, there's nothing that special about a 50/50 split, so this isn't really a proof of anything. In fact ... well, I'd have thought that if you adjusted for goaltending, you would find that offense had a slightly *higher* spread than defense, not an equal one ... for roughly the same reasons as in the NBA. The difference in hockey should be lower: your superstar can't control the play in hockey as much as in the NBA, and Wayne Gretzky got only half the playing time that Michael Jordan did. But you should get a *slight* effect.
But we didn't control for everything. Even without the starting goalie, the "defense" SD still includes the effects of the backup goalie. Still another factor is that I used overall save percentage for goalie talent, without adjusting for power-play shots. That factor probably overstated the goalie SD a little bit.
The biggest factor we didn't control for is that teams vary a lot less in power play opportunities than they do in *opposition* power-play opportunities (that is, some teams take a lot of penalties). Maybe that makes up the difference. Maybe my gut feeling is still correct, that Phil Espositos are generally more important than Harry Howells.
I didn't look at football at all, so this allows the theory to make a prediction. Obviously, the QB is the most concentrated talent position on the field, and it affects offense the most. So, after adjusting for possessions and field position, the SD of points scored in the NFL should be significantly higher than the SD of points allowed. Anyone got that data?
And are there other sports than hockey where defense varies more than offense?