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Explanation

History

Wales switched to the Elo system in 1972, soon after it was adopted by FIDE.
The original version was designed with assistance from Professor Elo himself.
Changes have been made over time - not always for the better.

G1 Ratings

You start the season with a G1 grade. This is used all year for Tournament entry etc.

G2, G3 ratings etc.

For rating your games are divided into batches.
Each has a different grade, G1, G2, G3 and so on.

Batches

A batch consists of at least 16 games but doesn't end during a tournament.
So John Waterfield's first batch contained 20 games - 15 plus 5 at Caerleon.
He will now start a new batch with rating G2=1981 instead of G1 = 1967.

Inserting one late result can set off an avalanche of changes.
For instance, John would start his new batch 5 games earlier, with a different G2.
I must have results as they happen.

Batches are too small

Players are supposed to reach their true rating at the end of a batch
Batches should contain at least 800/K games, say 60 for K=15.

K-Factors

Your K-factor determines how quickly your rating changes.
It should be 35 for your first 150 games, then 25 for the next 150. After that it falls to 15.

The Welsh system is slow

For ordinary players Wales appears to have the slowest system in the entire world.
The USA is typical. K=15 above 2100 and 32 below

Expected Score

For each game you have an expected score.
For instance, you should score 30% against players rated 150 points above you.
So your expectancy is 0.3. If you draw (0.5) you have gained 0.2.
This is multiplied by your K-Factor.
So your rating increases by 3, 5 or 7 points depending on that.

Calculating expected scores (mathematicians only)

Use a normal distribution with standard deviation equal to 200 times the square root of 2.

The Table

In the table the four last columns show -

Total points for the batch
Expected score for the game
Total expected for the batch
Your new rating

Reform

The Welsh Grading Committee is expected to meet soon for the first time in a decade or two.
I hope it will tackle the anomalies listed above.