Saturday, 3 May 2014

IMPs to VPs

This is a summary of the 2014 update to the World Bridge Federation suggested scoring for Teams matches.

In a Teams Tournament each match is scored in IMPs, then based on the IMPs each team is awarded some Victory Points (VPs). This is done to level off the effect of a big IMPs win, and make every match in the tournament worth roughly the same. Although no one thinks of it this way, this is also what happens in football matches when the margin in goals is converted to a number of points. In football this is done in a very extreme way, and the margin of victory is basically ignored and you get 100% of the possible points if you have any positive margin of goals. In Bridge it's done more smoothly, and big IMP wins do translate into bigger VP wins.

The reason I'm mentioning this is because recently the WBF have adopted a new IMPs to VPs conversion table, which differs in two ways. Firstly, every different IMP win now scores a different number of VPs, which leads to some decimal VP scores. Secondly. there is a bigger reward for small wins, as noticed by Paul Gipson noticed this in his blog post:Changing Tactics.

The graph below shows how IMPs are converted to VPs, on the old and new scales. The vertical axis measures the winning margin as a percentage of the maximum win.

Sources: New Scale and Old scale

You can see the new scoring system is much smoother, and is 'higher' on the left hand side, meaning small IMP wins now have a bigger effect than before. The new scale also flattens off a bit sooner, meaning (for a 16 board match), you get a 100% win at 60 IMPs.

Note (October 2015): Following some interest from a Swedish bridge forum (in Swedish) I've had another look at this. My graph is correct, although the situation is slightly blurred by the fact that the old scale was out of between 25 and 30 (a whitewash was 25-0, a draw 15-15) and the new scale is always out of 20. Because of this I've had to use 'VPs gained over loser as % of max' as my measure. This slightly exaggerates the effect of the new scale, where a narrow win like 11-9 on the new scale is counted as more significant than a narrow win like 16-14 on the old scale.

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