IMP Scoring at Pairs
Use of IMP scoring in a pairs event gives people a chance to practice Teams tactics.
We use Cross IMP scoring. For this, the NS pair at Table 1 compare scores on Board 1 with the EW pair at Table 2 to give an IMP score. This is repeated by comparing scores with the EW pair at Table 3, etc, and these IMP scores are then averaged to give the overall IMP score for NS pair 1 for Board 1. This process is repeated for each board for NS pair 1, and then for each pair.
Many years ago, events were scored by hand. Since Cross IMP scoring involved a disproportionate amount of effort for hand scoring, an alternative approach called Butler scoring was developed to simulate teams scoring at pairs but with less effort for the scorer. Both approaches give similar results. We have opted for Cross IMP scoring rather than Butler scoring.
How to Interpret the Scores
A score of zero means that (on average) you have matched the scores from the other pairs. This is clearly the equivalent of 50%.
The EBU NGS system includes an algorithm to convert Cross IMP scores to equivalent percentages. The result of this is that a score of 1 IMP per board (i.e. 27 IMPs if you played 27 boards) is equivalent to 6.5%. So +27 IMPs is equivalent to 56.5%, while -27 IMPs is equivalent to 43.5%.
IMP Scoring Tactics
The tactics for an IMP scored event should be the same as for a Teams Match, and they differ from Pairs tactics in a number of ways.