Going to play my first international tournament next week.
As this is my first I do not have a FIDE ELO (I have national rating however).
How will my games be rated and how is my expected score for each game calculated?
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The FIDE Rating Regulations give details of how the system works. You should scroll down to section 8. The working of the FIDE Rating System which describes the mathematical tables and formulae used for calculating ratings.
Note that a fudge factor called the "Development Coefficient" is used in calculating ratings which determines by how much new results affect your existing rating once you have one.
K = 40 for a player new to the rating list until he has completed events with at least 30 games
K = 20 as long as a player's rating remains under 2400.
K = 10 once a player's published rating has reached 2400 and remains at that level subsequently, even if the rating drops below 2400.
K = 40 for all players until their 18th birthday, as long as their rating remains under 2300.
As a new player you will have a high K which will mean that your rating will change rapidly if you have results which vary widely from your previous results.
If you're unrated, you don't really have an expected score. How can anything be expected when your rating is totally unknown? But the win expectancy table is sometimes used in reverse, to calculate the rating of an unrated player based on the actual score.
There are actually a few different formulas that might be used to determine your initial rating, depending on the type of tournament and your score, according to the FIDE Ratings Regulations:
8.2 Determining the Rating 'Ru' in a given event of a previously unrated player.
8.21 If an unrated player scores zero in his first event his score is disregarded.
First determine the average rating of his competition 'Rc'.
(a) In a Swiss or Team tournament: this is simply the average rating of his opponents.
(b) The results of both rated and unrated players in a round-robin tournament are taken into account. For unrated players, the average rating of the competition 'Rc' is also the tournament average 'Ra' determined as follows:
(i) Determine the average rating of the rated players 'Rar'.
(ii) Determine p for each of the rated players against all their opponents.
Then determine dp for each of these players.
Then determine the average of these dp = 'dpa'.
(iii) 'n' is the number of opponents.
Ra = Rar - dpa x n/(n+1)
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%
8.24 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)
Let's say you play in a Swiss and lose to a 2000, beat an 1800, draw to a 1900, draw a 1750, and lose to a 1950.
The first step is to calculate the average rating of your opponents. In this example, it is 1880. Since you scored less than 50%, we then calculate dp using table 8.1a (which is at the link.) You scored 2/5, which is 40% or 0.4. According to the table, this gives a dp of -72. So your rating for this tournament would be 1880 - 72 = 1808.
When your opponents are calculating their own ratings, this applies:
8.52 If the opponent is unrated, then the rating is determined at the end of the event. This applies only to round-robin tournaments. In other tournaments games against unrated opponents are not rated.
So your games against them would not count towards their ratings, in this case. But if this was a round-robin instead, then your rating would be calculated first (using the slightly more complicated process for round-robins described above), and their win expectancy against you would then be determined based on your new rating.