Recently someone said at the table that they didn't understand how matchpoints work (in duplicate bridge) so i thought I would find out. Matchpoints are what you see in the results under the headings MP and Matchpts.
Essentially they are a score based on your relative position or rank within your peer pairs. They are not proportional to your raw score for the board (eg +620 for making 4H vulnerable or -50 for going down by 1 trick), so for example if you are top board by being the only one to bid and make a slam and so have a very high score, you will get the same matchpoints as when you have a score only just higher than the second board. The same thing applies in reverse at the bottom when one crashes and burns. A passed in board gets a raw score of 0 for both pairs.
At the end your matchpoints from all your boards are used to calculate your overall % (the sum of your matchpoints as a % of the maximum possible - which would be if you were solo top board on every round). You can see the maximum in Pianola.
The number of matchpoints available per board depends on the number times that the board has been played. If two or more pairs have the same raw score, then they get the average of the matchpoints for their positions.
This can be illustrated by an example:
Suppose there are 7 rounds and no tied scores. Top board will get (7-1)*2 = 12 matchpoints , second (7-2)*2 = 10 matchpoints and so on down to sixth with 2 then bottom/seventh with 0.
If say 2 pairs get equal top board, then the matchpoints for first and second places are averaged and they each get (12 + 10)/2 = 11. Third board still gets 8 matchpoints.
If say 3 pairs tie for second board then top board will get 12 and the 3 pairs will each get (10 + 8 + 6)/3 = 8 matchpoints. Fifth board still gets 4 matchpoints.
You will also see in the results your % for each board. This is calculated as the pair's matchpoints as a % of the highest possible matchpoints for the board.
So in the first example, top board will be 100% (12/12) and in the second the two equal top boards will each be 92% (11/12). Third board will be 67% (8/12) and fourth board 50% (6/12) in both cases.
In the third example, top board will be 100% and the three equal second boards will each be 67% (8/12). Fifth board will be 33% (4/12).
The way to think of the per board % is not as an exact indicator of your position/rank in that board (see example 2 where you were =1st but not 100% and example 3 where you were =2nd but only 67%) but of the board's contribution towards your overall result or how 'well' you did (eg example 3 where your 8 matchpoints were 67% of the possible maximum). A % > 50% means you got more than the average matchpoints (6 in these examples).
In example 3, your % is lower than you might have expected because two other pairs did as well as you so you got 8 not 10 matchpoints. Or consider this: if every pair gets the same score, they will all be equal top board and also equal bottom; so it is a flat board and everyone gets the average: 6 matchpoints and 50%.
The % is also sometimes shown in the Bridgemates after the score has been accepted - EG NS 25% and EW 75%. Of course this % only reflects the rounds that have been played so far.
Sample screenshots from Pianola results. NB these are based on a small session where each board was only played 3 times. This pair played 24 boards so the maximum possible matchpoints are 24*4 = 96. They got 43 and so 44.79%. They were playing EW so for board 7 were bottom board and for 8 were equal top board.
