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Old 01-12-2013, 10:48 PM   #95
krosero
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I'm curious as to whether you think the issue implicit in the bolded section of your post undermines the usefulness of AM as a means of measuring quality of play. It seems to me that in matches where both players regularly attack the net, their respective AMs will tend to be higher than those of players who stick more to the baseline. The reason for this, as you alluded to, is that when a player approaches the net he takes away any opportunity for his opponent to hit an unforced error, because attempted passing shot errors are counted as forced by convention. This effectively halves the probability that an unforced error is hit during a point where one player is at net when compared to a point where both players remain at the baseline, as in the former only one player can potentially make an unforced error, while in the latter either player can. This fact is important because any reduction in unforced errors necessitates a corresponding increase in the fraction of points won by aggressive plays, and hence an increase in AM.

Was the Sampras-Stich match really as high quality as the AMs make it out to be, for instance? I have my doubts; it seems that these figures might, to some extent, be an artifact of the playing styles of Sampras and Stich.

I feel this is an issue, not least of all because I see no reason to think that matches involving lots of serve and volleying or lots of chip and charging are of higher quality in general than matches played primarily from the baseline.

Do you have any thoughts on this?
Yes I think this is an issue with any stats that use Unforced Errors: namely methods like the Aggressive Margin, winner/error differentials, ratios, etc.

On grass you will commonly see very high winner/error differentials: +50 or more. On clay it's hard to get even a positive differential, and negative ones are common.

Perhaps I should have listed the AM's grouped by surface, because that is the factor that seems to have the greatest impact on how people play -- and therefore an impact on how UE's are scored in those matches.

On grass two SVers coming in behind every serve will have very few opportunities to make UE's. On clay two baseliners engaged in a war of attrition are going to have many such opportunities.

That's one reason that AM's are higher on grass. But leaving aside the question of playing styles, it's also just generally true that you can make more winners and forcing plays on a fast surface. That has nothing to do with styles per se, because you could have two baseliners playing each other, and they will find it to be true: they will have an easier time forcing their opponent when they're playing on a fast surface.

On clay you can hit the ball just as hard, or just as wide, but the defender gains an advantage, as compared to grasscourt matches. Any surface that slows down the ball will make it easier for the defender to catch up to it, and harder for anyone to hit winners or to make forcing plays.

And that's what the AM is about: making forcing plays while keeping your unforced errors down. That is just inherently easier on a fast court.

So that makes it really impossible to compare AM's -- or winner/error differentials and ratios -- across surfaces (except for limited purposes like I did with the Federer-Delpo matches).

Take the top AM's in my opening post as examples. McEnroe has 53% on grass, the highest AM of all. The highest AM on clay belongs to Nadal, with 35%. No one would suggest that McEnroe's level on grass is a greater level than what Nadal achieved on clay. I think it's fair to assume that both of those performances were GOAT level for their respective surfaces.

Sampras/Stich should be compared to other grasscourt matches of the time period. It was very common then for players to follow all, or nearly all, of their serves to net. Sampras/Stich could easily be compared with Sampras/Becker, for example.

But if you try comparing it to this era's matches at Wimbledon, which commonly feature two baseliners slugging it out, you're going to have problems.

At the very least you could not use the numbers in such a comparison literally, if at all.

But that all results from surface being such a large factor in tennis. I'm not sure there's any good or efficient way around this. I, for one, would not want to chuck the distinction between a forced and an unforced error; I would just restrict comparisons by surface.

But maybe we need to think more about how to score UE's.
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