Bridge Handicaps Explained

XG GRADES EXPLAINED

INTRODUCTION
XG Grades are an adaptation of the National Grade System (NGS) implemented by the English Bridge Union. A National Grade (NG) is an assessment of the playing standard of all affiliated members. Full details of the EBU NGS is available here.https://www.ebu.co.uk/documents/miscellaneous/ngs/full-guide.pdf

NGS CONCEPTS AND CALCULATIONS
A players NG is based on their club and tournament results over their last 2000 boards played. For the typical player, this amounts to about 80 sessions.
The NG is a weighted average calculation where recent results have a higher weighting than older results.
“Average” players will have an NG of around 50%, and more experienced players will have higher NG, say 55% to 70%.
After every session, the NG is re-calculated for all participants in that session. The calculation is based on a comparison between the actual scored percentage and each of the players Expected Score.
The player's Expected Score (ES) is based on
The players own current NG.
Their partner's current NG.
The average current NG for all the pairs they are competing against.
An NG is calculated for all individual players and for pair combinations.
The NG is expressed as a percentage that range from about 40% to 70%
Any player must play a minimum of 150 boards before their NG is valid.
Any session results more than 3 years old are excluded from the calculations.
NGS INCLUDED SESSION TYPES
Included session types are

Duplicate pairs events scored as match points or IMPs.
Multiple teams events scored as match points or IMPs.
Swiss Pairs events scored as match points or IMPs.
Swiss or Round Robin Teams events. This is based on the pairs cross-IMP Butler result.
 
EXAMPLES
Example 1
Assume …
your current NGS = 55%
your partner’s current NG = 61%
the opponents average XG = 52%

In this session, the Expected Score is 50%
plus Average of you and your partners current NG
less Average of the opponents NG

= 50 + (55 + 61) / 2 - 52
= 56%

If the pair scored 57% for the session, the partnership has scored 1% (57 less 56)
ABOVE expectation, so this has a positive effect on your NG.

Example 2
Assume this pair is playing in a much stronger field where the opposition's
average XG is 60%
your current NGS = 55%
your partner’s current NG = 61%
the opponents average XG = 60%

In this session, the Expected Score is 50%
plus Average of you and your partners current NG
less Average of the opponents NG

= 50 + (55 + 61) / 2 - 60
= 46%

If the pair scored 57% for the session, the partnership has scored 11% (57 less 46)
ABOVE expectation, so this has a very positive effect on your NG.

Example 3
Assume this pair is playing in a much weaker field where the opposition
average XG is 47%
your current NGS = 55%
your partner’s current NG = 61%
the opponents average XG = 47%

In this session, the Expected Score is 50%
plus Average of you and your partners current NG
less Average of the opponents NG

= 50 + (55 + 61) / 2 - 47
= 61%

If the pair scored 57% for the session, the partnership has scored 4% (57 less 61)
BELOW expectation, so this has a negative on your NG.

Bottom line

If you play above expectations, your NG will increase. Conversely, if you play below expectations, your NG will decrease.
If you play with a good player and play poorly, your NG will decrease
If you play with a good player and play quite well, your NG will probably not change
If you play with a good player and play very well, your NG will increase a little
If you play with an inexperienced player and play quite well, your NG will increase
If you play with an inexperienced player and play very well, your NG will increase
As the NG recalculations are dependent on many factors, it is difficult to predict how the NG changes from session to session. A rough rule of thumb is that, for every 1% variation from the expected score, the XG will change by about 0.025% (give or take).

XG GRADES
XG Grades are calculated for all participants is the weekly XClub competitions. Some facts and figures:

Approximately 45 NZ clubs, and 4000 players participate in the XClub competition.
All participating clubs play the same hands in each session.
The participating clubs can use any of the scoring system currently used is New Zealand ie Compass or Scorer or CompScore2.
The across-the-board result is calculated for each session and it is this score that is used in the XG calculations.
XG Grades are calculated for individual players only (ie XG Grades are not calculated for pairs)
The new XG Grades are calculated at the end of each week and published on the XClub website and on this website.
USING NG GRADES FOR CLUB HANDICAPS
XG (and NG) grades are perfect for club handicapping. There are two options for the applied handicaps:

Simply use the players NG
Use the pairs Expected Score (ES)
Option 2 is preferable as it considers the playing strength of the competing field. For example

If an average pair is playing in an average field, their ES will be about 50%
If an average pair is playing in a more experienced field, their ES will be less than 50%
If an average pair is playing in a less experienced field, their ES will be higher than 50%
It is possible that the strength of the NS pairs is significantly different from the strength of the EW pairs. ES handicapping takes this into account.
The calculations for Expected Score handicapping is more complicated as the current NG Grades is used required for all participants in any session. The Compass Scoring System does these calculations automatically.

Bob Fearn
October 2020