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Yorkshire
Chess Association |
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The Meaning
of Grades |
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Chess grades are an attempt
to measure the relative playing strength of players. They are based on past results and are thus a measure of average
performance over those games, which may span a period of one, two or even three
years, rather than a measure of current "form".
Qualitatively, the higher the grade the greater is the supposed
strength. Thus the higher graded of two
players is presumed to be the stronger in average performance against the same
opponents.
Quantitatively, the difference between a player's grade and that of
an opponent is the predicted difference from 50% in the player's score, over a
number of games, against players of the opponent's grade. Thus if a player graded 135 played an number
of games against opponents graded 115, then that player would be expected on
the basis of the grades to score 70%.
This is fairly clearly related to the way a player normally scores his
opponent's grade plus 50 grading points for winning a game, and scores his
opponent's grade in grading points for a draw.
A difference of more than 50 points does of course mean the same as a
difference of 50 points. (See Problems Caused by Simplification)
These predictions are not
related to the individual games results of win, draw or loss. In the above example the player graded 135
could achieve his 70% with +7=0-3 (i.e.7 wins, no draws and 3 losses), or with
+6=2-2, or with +5=4-0. A player who
liked to press, albeit speculatively, for wins would be more likely to score +7=0-3,
whereas the cautious player whose first priority was to avoid losing would be
more likely to score +5=4-0. The
grading system does not seek to address such differences.
Importantly, grades offer
quite a poor basis for the prediction of the result of a single game, if the
players are quite closely matched. In a
single game you can score only 100%, 50% or 0%. So what does a predicted score of 70% mean in a single game? You'd be looking for an answer like "an
x% chance of a win for the higher graded player, a y% of a draw, and a z% of a loss". Such a prediction regarding a single game is
beyond the scope of the grading system.
There is nothing in the
grading system which says how much higher graded than your opponent you need to
be in order to be expected to win.
Superficially, a difference of more than 25 points would suggest that a
win for the higher graded player was more probable than a draw, as the chances
would be nearer 100% than 50%, but that is rather simplistic. An analysis of actual results in comparison
with grade difference could throw light on that question. There is also to be considered the disparity
between White and Black which is assumed to average out over a number of games,
but which is generally accepted as slightly tilting the odds in a single game
between otherwise equal opponents.
Predicting team match
results is, on the other hand, quite
feasible. You total each team's grades,
divide the difference between the two totals by the number of boards, and the
result is the difference from 50% in the expected game point totals. Limiting the difference in grades used on
any given board to at most 50 will improve the prediction.