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Cross‑IMP Scoring

Friday Night

For many years the Young Chelsea scored its Friday night game in IMPs according to the “Butler” scoring method (named after Geoffrey Butler, a former president of the now‑defunct British Bridge League, who devised the method as a means of scoring British international trials). Your score on any particular deal was compared with a “datum”, which was established by discarding the top and bottom one or two scores achieved across the field on the deal and averaging the remaining scores. The difference between your score and the datum was converted to IMPs plus or minus according to the normal scale.

 

With the widespread use of computers in scoring bridge events, the Butler method has largely been superseded by Cross‑IMPs in serious competitions. For Cross‑IMP scoring, each score is compared with every other score on the same deal—you score‑up in IMPs with every other result achieved on the deal, and then take the average of all those IMP scores. This has the advantage of not discarding any valid scores to achieve the result, and fortunately the computer does the extra work for us.

 

Since April 2006, Friday night club games have been scored by Cross‑IMPs. Your score on any particular deal is compared with those of every other pair sitting in the opposite direction to you, across the field. The total of these comparisons in IMPs is averaged to give you your score on the deal.

 

The effect of this scoring is that you should be using teams rather than pairs tactics throughout the competition, e.g., securing your contract rather than worrying about overtricks.

 

 

Converting to Percentages for Ladder Purposes

Friday IMP scores are converted to percentages for inclusion in the Monday‑Wednesday‑Friday ladder database. The basis used in the YCBC computer system, founded on observations going back over 20 years, is that

  • 60% is equivalent to +42.5 IMPs
  • 70% is equivalent to +85 IMPs

 

These figures are based on playing 24 deals, the original Young Chelsea norm for Friday nights. However for some years we have now played an extra round on Fridays. So now when we publish an IMP result on Friday the figures need to be factored to take account of the extra deals played.

 

Here are the calculations to convert your score to the equivalent 24‑deal percentage for Ladder purposes:

 

Based on and factored‑up to 26 deals played

  • 60% is equivalent to +46.0417 IMPs
  • 70% is equivalent to +92.0833 IMPs
  • 0.2171946% corresponds to 1 IMP
  • You can use this table to convert your IMPs to their percentage equivalent when 26 deals were played

 

Based on and factored‑up to 27 deals played

  • 60% is equivalent to +47.8125 IMPs
  • 70% is equivalent to +95.625 IMPs
  • 0.20915% corresponds to 1 IMP
  • You can use this table to convert your IMPs to their percentage equivalent when 27 deals were played

 

Based on and factored‑up to 28 deals played

  • 60% is equivalent to +49.5833 IMPs
  • 70% is equivalent to +99.1667 IMPs
  • 0.20168% corresponds to 1 IMP
  • You can use this table to convert your IMPs to their percentage equivalent when 28 deals were played

 

 

Archived Results—The Scorebridge Effect

For older results, accessed via the archive page, we used to use, as our scoring program for pairs events, Scorebridge, which produced extremely attractive results output for the web. However, in two‑section events it factored up the scores to match the greater number of deals played by either section, so you need to take that into account on nights when two sections were in play, if you wish to convert your score to its percentage‑equivalent.

 

E.g., if you played 26 deals and your archived result on the web showed a score of 56 IMPs on a night when the other section played 28 deals, you need to convert your score, as displayed on the web, according to the formula for 28 deals rather than that for 26 deals. So, you calculate

 

50% + (56 × 0.20168%) which gives a result of 61.29%.

 

However, if there was only one section, or both sections played the same number of deals, you simply consult the table that applies for the number of deals you played.

 

 

Page last updated 23 August 2019

ONE OF THE WORLD’S GREAT BRIDGE CLUBS