EBU National Grading Scheme

[TimG]

Player A and B are partners; they play together regularly and do quite well, both acquiring ratings of 57.00 or so. However, player A is completely dependent on player B and when A plays in a pickup partnership with C he does terribly, there is no "gelling" (to use the term in the NGS guide) and they almost always come in last. Player A then plays more and more with player B, getting his grade back up to 56/57 and then plays with C again with the same disastrous result.

I do not know the details of the EBU National Grading Scheme, but in general a dynamic rating system should work this way. . .

 

When A plays with C and his rating drops, then goes back to play with B and has the same results that previously had the partnership at 57, A's rating will not go all the way back up to 57. A's rating will end somewhere below 57 and B's rating will end up somewhere above 57.

 

I don't think the scenario where A plays like a 57 player with B, but plays like a last place player with C is realistic. Maybe A unsuccessfully plays a wild and gambling game when he plays with C? If that is truly the case and A, B, and C seldom play with other players, then the rating system could give some odd results for these players. But, as A, B, and C play with other players, these oddities should be lessened.

[gnasher]

I do not know the details of the EBU National Grading Scheme, but in general a dynamic rating system should work this way. . .

 

When A plays with C and his rating drops, then goes back to play with B and has the same results that previously had the partnership at 57, A's rating will not go all the way back up to 57. A's rating will end somewhere below 57 and B's rating will end up somewhere above 57.

The EBU's system does do that. The contribution of a single result to each player's grade is:

0.5 * [own grade] – 0.5 * [partner's grade] + [score adjusted for strength of field/opponents]

If A has dropped to 55, B is still rated 57, and they score 57, the contributions from this game will be:

A : 56

B: 58

Curiously, last night somebody told me that this characteristic was a flaw in the scheme.

[mike777]

I dont get any of this?

 

what is the point?

 

so far no one says any point.....

 

 

 

If you have a claim then say so

prove why it matters

prove why we care

prove your method best

 

 

so far all i see is alot of crap

[TimG]

The EBU's system does do that. The contribution of a single result to each player's grade is:

0.5 * [own grade] – 0.5 * [partner's grade] + [score adjusted for strength of field/opponents]

If A has dropped to 55, B is still rated 57, and they score 57, the contributions from this game will be:

A : 56

B: 58

Curiously, last night somebody told me that this characteristic was a flaw in the scheme.

I think there is probably need for some modification to reflect partnership experience. In general, a pickup partnership of two 57 players should not be expected to perform as well as an experienced partnership of two 57 players. Of course if the two 57s in the pickup partnership earned their ratings by playing mostly in pickup partnerships while the experienced pair earned their 57 ratings almost exclusively playing with each other. . .

 

Anyway, I would seem to me that the partnership aspect of things is more complicated that assigning rating based upon the arithmetic mean of two ratings. A case can be made for having partnership ratings only rather than individual ratings.

[awm]

There are a lot of issues with these ratings schemes, some of which are hard to resolve. The "experienced partnership" issue is one problem; presumably I could raise my rating by playing mostly in experienced partnerships (which tend to do better than the sum of our ratings) rather than pickup partnerships (presumably vice versa). There are also some issues with non-linearity of expected score... it's very hard to get scores above 80 or below 20 for example regardless of how well you play and how weak/strong the field is. And of course players who have few regular partners or play mostly in team events are hard to rate.

 

However, the right approach might be to compare this system to what preceded it (just master points) instead of to an "ideal system" of some sort, and I think there it comes out fairly well.

[mikestar13]

My take is that ratings are of little or no value in differentiating skill levels of advanced and expert players, though somewhat useful for lesser players. I don't need a number to tell me that Meckwell or Fantunes are better than Joe Schmo and his brother Moe, and the best way to rate Meckwell vs. Fantunes is to have them play each other. In baseball, the World Series is won by the team that wins 4 out of 7 on the baseball field, not by the team with the better stats. If you are a good enough player to know that even good rating systems such as the EBU's are deeply flawed, you are too good to need them anyway. For the rest of us (in ACBL terms, substitute your own countries' "currency"), 500 Master Points and a couple of dollars will buy you a cup of coffee (I've played in the days when it was 300 and a buck, some inflation, huh?)

[mr1303]

Is it important? No, not really.

 

It is interesting? Yes. I check my placing at least once a week.

 

Does it show any significant information? No, not really. My highest ranking was an ace of hearts, when I had little to do except play bridge and I was full of energy when I played. Now I have a job, my ranking has gone down a little.

[mchristie]

Thanks for the positive feedback, to those that have given it, thanks to Gnasher for reading the Guide so fully and explaining it here.

Some more thoughts from the developers...

 

Improved data on web pages? Yes it'll come sometime, but is dependent on other unrelated EBU software changes.

 

More emphasis on partnership grades? Yes, this is again a display (and search) problem. I'd like to show all partnerships where at least one player belongs to county X, and maybe all partnerships (with over 300 graded boards in the last 3 years) for player P. There are boring software constraints that make these more work to implement than they should be.

 

Non-linearity of partnership performance... How well should we expect a 64/48 partnership to perform, if not half way between the two grades? I doubt anyone knows, and doubt we could ever get enough data to test any theory. I wouldn't worry about pairs who are so strong/weak their score in a session should be over 70% or under 30%; they are rare and will probably get a realistic grade when they play in a session more suited to their strength.

 

Pick-up partnerships play worse than regular ones. There is now enough data to quantify this, if we had time to analyse it. An ad-hoc rule that reduced the expected score by x% for a partnership that had played at most 1 prevoius session in the last 3 years would work and not be hard to implement.

 

The real issue with the A,B,C problem mentioned recently is when A stops playing with C, and FOREVER AFTER will have a lower grade than B. This feels wrong for a system that is supposed to reflect current playing strength, and would need another ad-hoc rule to fix it. The rule would be "if you nearly always play with B your grade is pulled towards B's grade".

 

All ad-hoc solutions have downsides, and currently our principle is to keep the scheme as mathematically clean as practically possible, so no ad-hoc fixes in the foreseeable future.

 

That's all for now folks.

[scarletv]

To mgoetze: do you have a link to the German version, please?

 

http://vu2109-rails.delta.railshoster.de/frame

This link is not working?

[MickyB]

The real issue with the A,B,C problem mentioned recently is when A stops playing with C, and FOREVER AFTER will have a lower grade than B. This feels wrong for a system that is supposed to reflect current playing strength, and would need another ad-hoc rule to fix it. The rule would be "if you nearly always play with B your grade is pulled towards B's grade".

 

I don't think this would be an improvement. The fundamental problem is that there is limited data to differentiate A from B. I don't think that [tending towards] ignoring the data that we do have is an improvement; Better to just hope that A and B occasionally play with other partners, thus the amount of data we have will increase.

 

I like the idea of reducing the par score for pick-up partnerships.

[Cyberyeti]

One thing that would affect me if I cared.

 

My normal partner and I were thinking of going down to the local club and playing Fantunes (which we don't know that well as opposed to our normal system) for a bit of a laugh. This will potentially score us a much lower score than our normal system having 2 potential consequences:

 

1. People playing us when we're playing Fantunes will do much better than they "should" and their ratings will rise, ours will drop.

 

2. People playing us when we're playing our normal system will be playing against a pair much better than their rating.

 

The fairest way round this would be to allow people doing this to opt out of the NGS for certain sessions, but I don't think we can do this without losing the rights to magazine points and masterpoints.

[WellSpyder]

One thing that would affect me if I cared.

 

My normal partner and I were thinking of going down to the local club and playing Fantunes (which we don't know that well as opposed to our normal system) for a bit of a laugh. This will potentially score us a much lower score than our normal system

C'est la vie! I had a go at playing Fantunes at the EBU Autumn Congress, knowing (like you it seems) that my partner and I didn't really know the system well enough to match what we would expect to score playing a more familiar system. Sure enough, I have dropped a grade in the NGS as a result. Never mind - it was fun, and it was also very refreshing to see that almost everyone we played also thought it was fun to play against a completely different system. No doubt my grade will go back up again in due course if it was a genuine reflection of my normal game, and won't if it wasn't.....

[gordontd]

C'est la vie! I had a go at playing Fantunes at the EBU Autumn Congress, knowing (like you it seems) that my partner and I didn't really know the system well enough to match what we would expect to score playing a more familiar system. Sure enough, I have dropped a grade in the NGS as a result. Never mind - it was fun, and it was also very refreshing to see that almost everyone we played also thought it was fun to play against a completely different system. No doubt my grade will go back up again in due course if it was a genuine reflection of my normal game, and won't if it wasn't.....

I agree with this attitude. Most of my bridge is played with random pick-up partners, and I could opt out as a host, but I decided not to when this first came in, and I've stuck with it - even getting 27% with a partner with dementia. My grading has jumped around a lot, but if I do get an unexpectedly good result with a low-graded partner, I get the full benefit of it.

[Cyberyeti]

I agree with this attitude. Most of my bridge is played with random pick-up partners, and I could opt out as a host, but I decided not to when this first came in, and I've stuck with it - even getting 27% with a partner with dementia. My grading has jumped around a lot, but if I do get an unexpectedly good result with a low-graded partner, I get the full benefit of it.

Perhaps I wasn't clear enough, I don't care about my own grade (or indeed know what it is), I was concerned about the effect of my artificially high/low grade (depending on what I happen to be playing) on the ratings of people who play me and who do care about theirs.

[Vampyr]

I like the idea of reducing the par score for pick-up partnerships.

 

I agree with this attitude. Most of my bridge is played with random pick-up partners, and I could opt out as a host,

 

It would be a simple change to allow pick-up partnerships other than the actual host to opt out. I told the developers before the scheme was implemented that this would be an improvement, and do not understand why they disagreed. Now they are considering it... LOL

[TimG]

One thing that would affect me if I cared.

 

My normal partner and I were thinking of going down to the local club and playing Fantunes (which we don't know that well as opposed to our normal system) for a bit of a laugh. This will potentially score us a much lower score than our normal system having 2 potential consequences:

 

1. People playing us when we're playing Fantunes will do much better than they "should" and their ratings will rise, ours will drop.

 

2. People playing us when we're playing our normal system will be playing against a pair much better than their rating.

 

The fairest way round this would be to allow people doing this to opt out of the NGS for certain sessions, but I don't think we can do this without losing the rights to magazine points and masterpoints.

 

One session (and especially the 2 or 3 boards you play against any other pair) will have little effect on ratings in the long run. Yes, your opponents may get a small bump and you may drop a bit, but it shouldn't be significant when mixed in with your 100+ sessions over a year.

[awm]

While the non-linearity issue is most obvious with extreme cases, it may have effects elsewhere too. Here's an easy test to see whether this is happening.

 

Hypothesis: If we take the sum over all games where predicted score > 50% of [actual score - predicted score] then the value will be negative. The higher the threshold (i.e. if we take all games where predicted score > 55%) the more negative the value. Conversely, if we take the sum over all games where predicted score < 50% of [actual score - predicted score] then the value will be positive.

 

The implication would be that the real distribution of actual scores is closer to the mean than a strictly linear system would predict. The argument about predicted scores > 70% makes clear that this must be happening to some degree; the interesting question is whether the effect is enough to substantially bias the method.

 

The effect of such a bias would be that a player who wants to raise his rating is best off playing in the hardest events possible (such that his predicted score is quite low, but his actual mean score is better). This might seem good, but if many players pursued this strategy it would tend to dilute the strong events (which often depend on some self-selection to keep the field reasonable and make them more fun for the best players).

[Vampyr]

This might seem good, but if many players pursued this strategy it would tend to dilute the strong events (which often depend on some self-selection to keep the field reasonable and make them more fun for the best players).

 

 

Most of the strong events require pre-qualification or selection, so this should not be a problem.

[gnasher]

It would be a simple change to allow pick-up partnerships other than the actual host to opt out. I told the developers before the scheme was implemented that this would be an improvement, and do not understand why they disagreed. Now they are considering it... LOL

I don't think they are considering that, or they haven't said they are anyway. Mike said that they could reduce the expected score for an unfamilair partnership, but I can't see any suggestion that they'd let such a partnership opt out entirely. He also said that they're not going to do it in the foreseeable future.

 

Edit: FWIW, I don't think one should adjust for irregular partnerships. Forming a good partnership is a bridge skill. It seems wrong to penalise people for success in this aspect of the game.

Edited October 30, 2012 by gnasher

[gordontd]

Vampyr, in post 165, you've taken part of my earlier post, edited it, and put it under someone else's post to which it had no relation, then drawn a conclusion with which I don't agree. Please don't do this.

[Vampyr]

Vampyr, in post 165, you've taken part of my earlier post, edited it, and put it under someone else's post to which it had no relation, then drawn a conclusion with which I don't agree. Please don't do this.

 

I didn't draw a conclusion; I was just quoting things that were relevant to my own comment for the sake of context. But I see now that your "I agree with this attitude" appeared very inappropriately under another post. I was careless and did not intend to mislead. I am sorry.

 

Anyway when I said I thought they were considering allowing people to opt out, I had misremembered an earlier post. I am disappointed, I guess because I don't entirely agree with gnasher that forming a good partnership is a bridge skill. If it is, then being adaptable and able to play with people of all different temperaments and levels of skill is one too, so perhaps they are both being rewarded and it is not a big deal.

 

Still I feel that as people play with widely varying combinations of regular, semi-regular and casual/pick-up partnerships, one metric does not really measure all there is to know about comparable skill levels. On the other hand, the rating is a measure of performance, not skill, so maybe it is OK. The thing that I worry about, as I have said before, is for casual partnerships to be made less attractive, because that would be damaging both to bridge culture and to attendance numbers. There is no way of knowing whether this is the case, but in any case this is why I am interested in changes that might mitigate the effect. Which may not exist.

[TimG]

Edit: FWIW, I don't think one should adjust for irregular partnerships. Forming a good partnership is a bridge skill. It seems wrong to penalise people for success in this aspect of the game.

I don't think a true and accurate rating system is supposed to penalize or reward anyone. It's supposed to simply be a decent predictor of results.

 

If you are good at forming a partnership and have done so, this will be reflected in your results and your rating.