Yes the AM would be dependent to some degree on the opponent. I think that must be true for almost any measurement in tennis; there are very few things that come to mind that don't depend to some degree on your opponent. Aces and double-faults come to mind; but once the ball reaches your opponent's racquet, you will have his shot to deal with (or his error to benefit from), and it's no longer completely in your hands, so to speak. It's like counting winners and errors (the most common stats): you can definitely hit more winners if you're facing someone clearly inferior to you; it will not be much of a challenge for you to hit winners past him and to keep from making errors yourself. That's why I like to use the AM's as a benchmark, like you say, in rivalries. If you keep the same two players constant the comparison is less problematic -- and that's especially true if you keep the surface constant, since surface speed has a great effect on AM's. It becomes more complex if you're comparing the AM's that two players produce, not against each other, but against the field. If you're asking why Djokovic and Federer produced similar AM's at this Australian Open, then it's definitely a question to ask, whether Federer's opponents were different in some way -- perhaps inherently superior, or inferior -- to Djokovic's opponents. If there's some large difference in the quality of opposition that Fed and Djok faced at this tournament, that would have an impact on the AM's that they produced. However it's another thing simply to list as many AM's as you can for the top players in their entire careers. They all play the same competition on the same tour, so you would expect that their AM's can be compared fairly. At that point, though, there are other issues, like style of play. Does style of play have an impact on AM's? Do AM's favor a certain style? The more I think about that question the more complex the issue seems to be. We've all been working here on the assumption that SV reduces unforced errors and produces high AM's, because if two players are constantly rushing to net against each other, the defender will always be hitting passing shots or making forced errors, not UE's. And it's true that you can find many matches at Wimbledon, in older eras, where two SV players faced each other, coming in behind every serve, and making very few UE's. Hence they have high AM's. But there's a twist here: at the most basic level, you get a high AM by keeping your unforced errors down. There's a real opportunity in that sense for players like Borg or Nadal, who are masters at keeping their UE's down, to post high AM's. You can force these men into errors, but they will rarely hand you a point for free. And with low UE's, their AM's have to be high. By the same token, an aggressive net-rushing player facing a great defender will have no choice but to make a lot of UE's: because they have to go for their shots if they're going to win; and in going for their shots they will make plenty of UE's. So it's not true that merely having an aggressive style will get you high AM's, while the grinders will have low AM's. That really depends on the matchups. It's true that two net-rushers facing each other tend to crowd out opportunities to make UE's and they will often post extremely high AM's; and two grinders facing each other will have nothing but opportunities to make unforced errors, so in such matches you will often see low AM's. But an aggressive player facing a grinder is a different story. I don't know that there is any inherent advantage there, as far as AM's go. The aggressor might easily make a ton of UE's errors, and post a low AM; while the defender might easily get a high AM by making almost no errors. Upthread we had a discussion about Nadal, because he seems to be the archetype of the grinder who hits a low number of winners and makes a low number of errors. I thought that if we looked at his winner/error differentials, we might find low numbers. But I have found the opposite to be true: when he wins his matches Nadal has a better winner/error differential than his opponents even when he's playing a mega-aggressive player. Nadal might make far fewer winners than someone like Verdasco; but he also makes far fewer errors; and if Nadal wins the match he will almost always have a higher winner/error differential than Verdasco. Just because Nadal's numbers of winners and errors are low, does not mean that he ends up with low winner/error differentials: or to put it another way, just because his winners and errors are low does not mean that he can't post a higher Aggressive Margin. An Aggressive Margin is just the margin of your forcing plays as compared to your errors. Your winners and errors might be low but all you have to do is keep your winners ahead of your errors better than the other guy does, and you win the match. The absolute figures don't matter here. Nadal can post 50 winners while Verdasco posts 100. The only important question is, if Nadal posts 50 winners while making 30 errors (+20), can Verdasco post his 100 winners while making no more than 80 errors? Can Verdasco, in short, keep ahead of his own errors by a margin of +20? If he can't, he will probably lose that match to Nadal. If Verdasco can only keep ahead of his own errors by 10, while Nadal is keeping ahead by 20, then Nadal will come out at the end having won more points: and that almost always means that Nadal will win the match. The higher Aggressive Margin, mathematically, always goes to the player who wins the most points. So why should there be an advantage there for either an aggressive style or a defender's style? All you have to do is keep ahead of your own errors better than the other guys does. If anything -- I'm not pushing this, but if anything -- someone with heavy topspin who is ridiculously good at keeping his errors down will naturally be posting high AM's. That's why, against expectation, Nadal posted AM's above 40% even at the 2006 Wimbledon. I mean, it's true that Nadal is caricatured as being nothing but a dirtballer with a forehand in his early days; in fact he could do a lot more; but even so, he's the classic archetype of a defender; and even in '06, before the more "aggressive" facets of his game (like his serve, or his volley) had matured, he was posting these ridiculously high AM's. On grass. How does something like that happen, if in fact the AM method favors aggressive players who are all about hitting winners? It's a complex issue, and the style question is a valid one, when it comes to AM's. But it's not straightforward at all. As far as Nadal goes, I think those numbers from the '06 Wimbledon already show that he was better on grass than he is often given credit for. But we would have to regard him as even an even better grasscourter than that, in '06, if we decided that the AM method really does make aggressive players look better: because then Nadal is posting these high AM's despite the method being inherently biased against him. I don't think we need to go that far, because I don't think it's been shown that the AM method is inherently advantageous to aggressive players.