[tournament-org] The Bar in general
Geoff Kaniuk
geoff at kaniuk.co.uk
Wed Nov 7 13:31:55 GMT 2018
A. THREE PEAKS
The 3P incident was interesting. I has a conversation with Bob after the
event and it transpired that initially Bob had a top group of 6 players
something like 5d,4d, 1k,1k, 1k,1k. After closing the register GoDraw
set the bar at 1k, as this provides a jump from 2 to 6 players, even
though the bar-depth exceeds 3.
At this point two more players pitched up: a 2d and a 1d. I am not sure
if Bob had already done the draw, but there would have been no problem -
just pair them and sort out the draw later after entering results. This
would have left the bar at 1k as it does not get re-computed.
I assume he had not done the draw, and may have tried to add these two
players. This would be refused by GoDraw because it had already worked
out the bar. Now you have to re-open the register, add the two players,
and close the register.
Now GoDraw sets the bar afresh. At 2d the bar depth is 3 but still not
enough players to exceed the number of rounds. So it leaves the bar at 4d.
I do understand the tension involved in getting the first round going,
so Bob did the right thing and carried on with the draw. But actually
there is a simple way to deal with this:
After closing the register,
If you do not like the bar,
Just change it!
There is a field for setting the bar manually and you can adjust it to
what you want. The philosophy underlying GoDraw is that it is a TD's
toolkit, not a TD's boss!
You cannot change the bar after doing the draw. The only thing setting
the bar does, is to specify the player's initial McMahon score. Again
there is a field in the register editor which allows you to change the
player's initial MMS and this of course adjusts the players current MMS
as well. By making this adjustment for all the players you want above
the bar, you achieve what you want!
B. THE BGA BAR TABLE CONDITIONS
Toby reproduces four conditions which an ideal bar setting should meet.
These are indeed aspirations. But on their own they do not provide any
information that is useful for setting the bar. A 5 round tournament
would require exactly 32 players above the bar to guarantee a unique
winner. Three Peaks had a total of 34 so almost everybody above the
bar?. It might be well worth while having a look at BGJ 173 - Finding
the Bar which discusses
C. THE BGA BAR TABLE VALUES
I have in the past tried to find some documentation on the origins of
the table and the best I got was that it is lost in the mists of time.
When the table was invented, people did not have a good measure of the
probability of win between players of different strengths. But now
through the data collected by EGD we do have a good model. This enables
us to simulate McMahon tournaments to assess the effect of different bar
settings and that work is cited in the BGJ 173 article.
The outcome confirms that you cannot set the bar on the basis of number
of rounds alone, but it depends on the nature of the distribution of top
grades. It leads to the following table which is a rough linear
representation of the extremes of the bar populations in the simulations:
rounds 2 3 4 5 6 7 8 9 10
low 2 3 4 5 6 7 8 9 10
high 11 12 14 15 16 18 19 21 22
In these simulations the bar was calculated by a Monte Carlo algorithm.
The bar depth never exceeded 3 and was very rarely equal to 3.
Toby said:
>Of course, the statement on bar depth being less than 3 assumes that
>the grades are "correct" in some ill-defined way.
The McMahon system relies on players entering with realistic grades, and
so in all my discussions I always assume that grades are consistent with
player strengths as found on our rating page.
Geoff
33 Ashbury Close, Cambridge CB1 3RW 01223 710582
On 06/11/2018 13:27, TobyManning via tournament-org wrote:
> Further issues with the Bar at the 3 Peaks...
>
> *Background*
>
> The 3 peaks is a 5 round tournament. The entry was 5d 5d 2d 1d 1d 1k 1k
> 1k 2k.... and Bob Bagot wanted to put the bar at 1 kyu. However, the
> software (Go Draw) put it at 5 dan, so in the first round the 2 5 dans
> played each other (and the 2d played a 1 d). With advice from Geoff -
> for which many thanks - the bar was restrospectively changed to 1 kyu
> for rounds 2-5.
>
> Incidentally one of the 5 dans lost to a 1 dan - it can happen.
>
> *Analysis*
>
> The BGA Tournament Handbook
> (http://www.britgo.org/organisers/handbook/tournament4) states (my added
> emphasis):
>
>
> /4.2////The Bar/
>
> /Because a player's starting score is determined by their grade, a
> player who was 7 dan would have a massive advantage and the best chance
> to win the tournament, as such a player would start with a very high
> MMS. To counteract this, and to give as many people as possible a
> reasonable chance of winning the tournament, players at or above a
> certain rank all begin at the same MMS. This rank is called the McMahon
> bar. For example, if the bar is set at 3 dan (which is an MMS of 2) then
> no player can start at an MMS of more than 2, no matter what his or her
> grade: 3-dans, and all players stronger than 3-dan, also start with an
> MMS of 2, and are said to start "above the bar". The position for the
> bar depends on the number of rounds to be played and also the entry at
> each of the higher grades. BGA recommended guidelines are as follows:/
>
> /3 rounds/ /4-8 players/
> /4 rounds/ /5-10 players/
> /5 rounds/ /6-12 players/
> /6 rounds/ /7-15 players/
> /7 rounds/ /8-18 players/
> /8 rounds/ /9-22 players/
> /9 rounds/ /10-26 players/
> /10 rounds/ /11-30 players/
>
> /These figures attempt to meet the following conditions:/
>
> * /There must be a unique winner. This sets an absolute upper limit,
> of 2//^r ////players above the bar, where r is the number of rounds./
> * /If there are too many above the bar, the tournament will end
> without all of the top players having played each other./
> * /If there are too few above the bar, these receive an unfair (and
> unnecessary) disadvantage./
> * /Higher graded players should not run out of even game opponents./
>
> /The McMahon System imposes two quite severe constraints on the pairing
> of players at each round. The first is the rule that there are no repeat
> games. This increasingly restricts the opponents of the stronger players
> in the later rounds. The second is the aim of pairing players on the
> same MMS, which provides the main pairing diversity in the early
> rounds.////
> //
> //These two pairing rules, together with the nature of the winning
> probability between players of different grades, provides quite an
> important ingredient in determining the position of the bar. //*It turns
> out that players in the bar group have at least some chance of winning
> the tournament if the difference between the maximum grade and the bar
> grade (the bar-depth) is less than 3*//, whatever the number of rounds
> [See BGJ 173 Finding The Bar]./
>
> /If you are using Geoff Kaniuk's GoDraw to create the draw,//*it will
> automatically set the bar according to the above table taking into
> acount the restriction on the bar depth*//. This is particularly
> effective in tournaments where the dan entry is very fragmented with
> possible gaps in the higher grades./
>
>
> Of course, the statement on bar depth being less than 3 assumes that the
> grades are "correct" in some ill-defined way.
>
> It is clear that what is happening is the restriction on bar depth is
> over-riding the recommended nuimbers in the Table. I believe that this
> is wrong, and that the Table should have supremacy.
>
> The Table is constructed on 2 simple principles:-
>
> a) The total should be less than 2**n (where n is the number of rounds -
> this ensures a unique winner
>
> b) The Tournament winner's opponents should all have started above the
> Bar (although one appreciates that this may not have been the case if
> the number above the bar is odd and the eventual winner is drawn down in
> their first game).
>
> In the 3 Peaks example,each 5 dan will play 5 opponents, of strength
> (probably) 5d 2d 1d 1d 1k, and if (for example) one of the 1 dans wins
> all 5 games, including beating both 5 dans, they shdol win the Tournament.
>
> Of course one can always play handicap games at the top end.... but that
> is a different debate.
>
> Toby
>
>
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