Maponos

According to law 78D of the WBF Laws of Duplicate Bridge, the conditions of contest (which include the method of scoring) may not conflict with other laws. Those laws specify that a score difference of 700 points is worth 12 Imps. As we have seen, datum (Butler) scoring awards as many as 16 Imps to a score difference of 700 points. This would seem to indicate that datum scoring is illegal in events run under the WBF Laws. Opinions?

For cross-IMP pairs play, it appears that BBO's software calculates a pair's score on a board by dividing their total cross-IMPs by the number of tables against which their score has been compared (one less than the number of tables at which the board has been played). While this seems like the right thing to do, it turns out that the correct result is obtained by dividing total cross-IMPs by the total number of tables (including that pair's own table). To see why, consider the case of only two tables. Suppose that one table makes 3NT for +400, and the other goes down one for -50. A difference of 450 is worth 10 IMPs, so one table will score +10 cross-IMPs and the other -10. Therefore the pair who made will score 20 IMPs more than the pair who went down, which is twice as many as they are supposed to score. Clearly we must divide by 2 to obtain the correct difference. It is easy to verify that this result generalizes to more than two tables. Of course, on BBO each board is usually played a large number of times, so it make very little difference whether you divide by n or n-1. But the correct formula is just as easy to calculate as the incorrect one, so why not get it right?