Revision of September Rating List

Mark Orr

This article part of the series: Rating Officer Reports 2008 - 1, 2, 3, 4, 5

This article part of the series: ICU Bulletins 2008-09 - 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12

The September rating list, first published on the 5th, has been revised and republished today (18th). 47 ratings out of 773 have changed with 44 increases (the largest of which was 37) and 3 decreases (the largest of which was 43). The full list of changes is at the end of the article. The reasons for the revision are explained below.

Why the Revision was Necessary

Updates to ratings of foreign players
The main problem was out-of-date ratings of foreign opponents who already existed in the database from previous tournaments. Their existing ICU rating (based mainly on an older FIDE rating) was used instead of their lastest FIDE rating. For established players, whose ratings don't vary much, this made little difference but for juniors with rapidly increasing ratings it meant that their strength was underestimated. Consequently, the players who benefited most from the correction were Irish juniors in competition against foreign juniors, i.e. the Glorney Cup and it's associated tournaments.
Misreported results
The other problem was a simple misreporting of two games from the Rathmines A versus Elm Mount C match in the Branagan Cup. Instead of the correct Burns 0-1 Dunne and Kennedy 1-0 Coyle, the opposite results had been sent in. Consequently, after the correction, Dunne and Kennedy had their ratings adjusted upwards while Burns and Coyle had theirs adjusted in the other direction.

The Curious Case of David Fitzsimons

Looking at the list of changes below, you'll see that there was a third player, David Fitzsimons, whose correction was negative, in addition to the other cases (Burns and Coyle, explained above). Since David was one of the Glorney players expected to have benefited from the FIDE rating corrections, such a large downward correction is counter intuitive at first sight. However, it is correct and the explanation is quite illuminating about how rating systems work.

The following table lists David's last two tournaments in the September list, giving before and after ratings, K-factor, expected and actual scores. Because David's Glorney result was underestimated in the first version of the list, his rating dipped after that tournament and, crucially, went below the 2100 threshold used to calculate the K-factor for his next tournament. Thus David's fabulous performance in the City of Dublin Masters (he came clear first) was further enhanced by a K-factor of 40 (instead of his usual 16).

Tournament Before Exp Act K After
Glorney 2102 3.66 3.50 16 2099
City of Dublin 2099 2.77 5.00 40 2188

Compare that with what happened in the revised list. After his opponents' FIDE rating corrections in the Glorney, his ICU rating went up (instead of down) and his K-factor remained at 16. Subsequently, he still gained in the City of Dublin, but far less, and his overall gain was not as great.

Tournament Before Exp Act K After
Glorney 2102 2.94 3.50 16 2111
City of Dublin 2111 2.85 5.00 16 2145

Step Functions

Why did a set of relatively small corrections which, for most players, meant either no change or a modest gain, produce such a large change in one isolated case? The answer lies in the way K-factors are calculated with rules of the following form:

if R < 2100 then K = 40 else K = 16

Such rules are easy for humans to comprehend but, mathematically, they are step functions. Unlike other functions used in Elo rating calculations (see [ART::3]), step functions are not smooth and are capable of large changes in their output (in this case K) given only small changes in their input (in this case R). This is what happened to David - a small adjustment to his rating (after the Glorney) made a big difference to what happened in his next tournament.

The current implementation of the ICU rating system is not very revision friendly, given the amount of work involved for the ratings officer. However, in a potential future reimplementation of the system, where revisions may be easier and therefore more common than they are now, it might be worth considering ways to dampen down the effects of sudden changes in K-factor.

Full List of Changes

Diff Old New Name ICU #
+3716751712Hugh Doyle10145
+3319521985Liam Normoyle6143
+3115821613Ray Dunne10204
+2817151743Mark Berney10328
+2720042031Jan Mueller5971
+2213351357Daniel Cashin6570
+2017761796Michael J. Kennedy3955
+1614541470Christopher Young6254
+1013931403Sarah M. Cormican6525
+920072016Sam C. Osborne6252
+819141922Oisin P. Benson5884
+817291737Raino Soikkeli6782
+821332141Ryan-Rhys Griffiths6897
+716561663Gerard Smith1568
+712011208Sarah Jane Hearne10544
+615701576Anthony Whalley7261
+616501656Gerard Buckley3075
+611821188John A. Cormican6523
+616611667Ronan Magee10470
+513951400Kieran O'Riordan10235
+516321637Vincent Bissett65
+414821486Cormac Roche1449
+415911595Tom O'Sullivan1344
+216911693John P. Aherne9
+216091611Melvyn King10214
+215331535Pat McEvoy10307
+125572558Alexander Baburin7085
+122752276Colm Daly295
+117791780Danny Mallaghan2610
+119992000David B. Murray4941
+115041505David Cormican6330
+110641065Eoin Moran10418
+119691970Gerard MacElligott1078
+120552056Gerard O'Connell1231
+118211822Ireneusz Mazur12155
+118081809Jack Killane740
+118301831John Healy4563
+119491950Killian Delaney5104
+117681769Lukasz Goralski10298
+121862187Mark Orr1350
+116421643Pat Fitzsimons1733
+116171618Paul Reilly6048
+119261927Peter Cafolla159
+117761777Stawomir Jurkiewicz10413
+113191320William Ffrench4570
-2413901366Greg Coyle261
-2417981774John Burns137
-4321882145David Fitzsimons6236

Created 2008-09-18 ◦ Last updated 2014-07-23 ◦ Editor MO

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