newbie ranking investigation =))

[yarmolinsky]

Hey guys,


I must admit IRB ranking system is a bit confusing for me. Even though I've already received excellent bits of clarification on certain aspects of it, the whole picture is still vague and unclear for me.
I've decided to ask my newbie question and to have it sorted all at once rather than keep bugging you with my "whys" and "how comes" =))


I will use IRB explanations as a pattern and recent game Tonga - Fiji 23.06.2013 as example.


Step One: Check pre match ratings
- Tonga 76.17
- Fiji 71.29


Step Two: Allow for home advantage
The game was in Japan, no home advantage


Step Three: Calculate the Rating Gap
76.17 - 71.29 = 4.88


Step Four: Check the possible Core Rating Changes
according to IRB diagram, Tonga rating should go down by approximately 1.5 and Fiji rating should go up by approximately 1.5.


Step Five: Apply weighting factors
No such factors noticed by me


Result:
after ranking has been updated, we have Tonga rating down by 1.4 and Fiji rating up by 2.27.


What bewilders me most - how, on earth, come that the rating change values are different??? 1.4 and 2.27? If they are supposed to be equal? Something like "+1.5" for Fiji and "-1.5" for Tonga?


At what stage had I screwed up? What is wrong in my understanding of the scheme?


Thanks a lot in advance =))

 

[ZeFrenchy]

Where did you get the 2.27 from? I think that's wrong.

The way I find it easy to calculate is, from the 4.88, divide by 10, to get 0.488. That is the gain/loss in points in case of a draw. For a win you add +1.00 and for a loss you substract the same.

I'm guessing Fiji won, so should go up by 1.488 and Tonga down by the same amount.

 

[yarmolinsky]

I'm guessing Fiji won, so should go up by 1.488 and Tonga down by the same amount.

That's exactly what I thought it should look like! But:

Where did you get the 2.27 from? I think that's wrong.

This value is the actual difference in Fiji rankings.
IRB World Rankings - 17 June 2013. Fiji - 71.29
IRB World Rankings - 24 June 2013. Fiji - 73.56

73.56 - 71.29 = 2.27. Fiji rating is up by 2.27 after game, while Tonga rating is down by 1.4 according to IRB chart. That is what I can't figure out - why are these values different while they are supposed to be equal? =))

 

[smartcooky]

That's exactly what I thought it should look like! But:

This value is the actual difference in Fiji rankings.
IRB World Rankings - 17 June 2013. Fiji - 71.29
IRB World Rankings - 24 June 2013. Fiji - 73.56

73.56 - 71.29 = 2.27. Fiji rating is up by 2.27 after game, while Tonga rating is down by 1.4 according to IRB chart. That is what I can't figure out - why are these values different while they are supposed to be equal? =))

Fiji played the USA (won 35-10) on the 19th of June (midweek) so the ranking points from that match will be included in the update.


The previous figures were for after the previous weekend. If you check, you will find USA went down by the same amount as the missing difference; 0.87



EDIT: Actually USA played two matches as well (lost 38-10 to Japan on the 23rd), so you will have to take that match into account as well

To get it right, you will have to calculate ranking points for USA, Tonga, Japan and Fiji

Use this site, it works well

http://www.lassen.co.nz/pagmisc.php

 

[yarmolinsky]

smartcooky I owe you a pint of old good ukrainian beer
Thank you, man. It looks so simple now, when I saw my slip-up))