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From the FIDE handbook

8.22 If he scores 50%, then Ru = Ra

8.23 If he scores more than 50%, then Ru = Ra + 20 for each half point scored over 50%

8.22 If he scores less than 50% in a Swiss or team tournament: Ru = Ra + dp

8.25 If he scores less than 50% in a round-robin: Ru = Ra + dp x n/(n+1).

What's the justification for the rules for scores of less then 50% being different then over 50%?

As of March 1, 2024 the FIDE initial rating calculations are now defined in a consistent way.

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What's the justification for the rules for scores of less then 50% being different then over 50%?

I don't think there is any good justification for that. There is also a difference in the calculations for <50% between Swiss and round robin tournaments. I think these rules were originally just transcriptions of the logic of the code used by FIDE in their rating calculations.

In the old days FIDE rated tournaments rather than games. In a way it made sense to have rules for rating round robins and for Swiss tournaments. Nowadays FIDE rates individual games and so these elements of the rules are obsolete.

FIDE are a bit slow updating some of these documents but they have changed the rules in the latest rating document. The document you quote is called FIDE Rating Regulations effective from 1 July 2017 till 31 December 2021 (with amendments effective from 1 February 2021).

The latest document is called FIDE Rating Regulations effective from 1 January 2022. That says:

8.2 Determining the initial rating 'Ru' of a player.

8.2.1 If an unrated player scores zero in their first event this score is disregarded. Otherwise, their rating is calculated using all their results as in 7.1.4.

8.2.2 Ra is the average rating of the player's rated opponents.

8.2.3 If the player scores 50%, then Ru = Ra.

If they score more than 50%, then Ru = Ra + 20 for each half point scored over 50%.

If they score less than 50%, then Ru = Ra + dp

Ru is rounded to the nearest whole number.

The round robin element has gone. So, they have got rid of one element of confusion and difference but not the one between an overall score of 50%, >50% and <50%.

I suspect that the "Ru = Ra + 20 for each half point scored over 50%" bit is just wrong. I don't believe they calculate initial rating like that any more. It is a relic from rating by tournament rather than by game. I think they just forgot to take that line out while they were removing "If he scores less than 50% in a round-robin: Ru = Ra + dp x n/(n+1)"

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