The Irish Chess Union

Revision of September Rating List

by Mark Orr (September 2008)

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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The main problem was out-of-date ratings of foreign opponents who already existed in the database from previous tournaments.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 ICU Rating Calculations), 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 #
+37	1675	1712	Hugh Doyle	10145
+33	1952	1985	Liam Normoyle	6143
+31	1582	1613	Ray Dunne	10204
+28	1715	1743	Mark Berney	10328
+27	2004	2031	Jan Mueller	5971
+22	1335	1357	Daniel Cashin	6570
+20	1776	1796	Michael J. Kennedy	3955
+16	1454	1470	Christopher Young	6254
+10	1393	1403	Sarah M. Cormican	6525
+9	2007	2016	Sam C. Osborne	6252
+8	1914	1922	Oisin P. Benson	5884
+8	1729	1737	Raino Soikkeli	6782
+8	2133	2141	Ryan-Rhys Griffiths	6897
+7	1656	1663	Gerard Smith	1568
+7	1201	1208	Sarah Jane Hearne	10544
+6	1570	1576	Anthony Whalley	7261
+6	1650	1656	Gerard Buckley	3075
+6	1182	1188	John A. Cormican	6523
+6	1661	1667	Ronan Magee	10470
+5	1395	1400	Kieran O'Riordan	10235
+5	1632	1637	Vincent Bissett	65
+4	1482	1486	Cormac Roche	1449
+4	1591	1595	Tom O'Sullivan	1344
+2	1691	1693	John P. Aherne	9
+2	1609	1611	Melvyn King	10214
+2	1533	1535	Pat McEvoy	10307
+1	2557	2558	Alexander Baburin	7085
+1	2275	2276	Colm Daly	295
+1	1779	1780	Danny Mallaghan	2610
+1	1999	2000	David B. Murray	4941
+1	1504	1505	David Cormican	6330
+1	1064	1065	Eoin Moran	10418
+1	1969	1970	Gerard MacElligott	1078
+1	2055	2056	Gerard O'Connell	1231
+1	1821	1822	Ireneusz Mazur	12155
+1	1808	1809	Jack Killane	740
+1	1830	1831	John Healy	4563
+1	1949	1950	Killian Delaney	5104
+1	1768	1769	Lukasz Goralski	10298
+1	2186	2187	Mark Orr	1350
+1	1642	1643	Pat Fitzsimons	1733
+1	1617	1618	Paul Reilly	6048
+1	1926	1927	Peter Cafolla	159
+1	1776	1777	Stawomir Jurkiewicz	10413
+1	1319	1320	William Ffrench	4570
-24	1390	1366	Greg Coyle	261
-24	1798	1774	John Burns	137
-43	2188	2145	David Fitzsimons	6236

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