[dbarto]

what is the difference in IMPs and MPs

[Mbodell]

The two most common forms of scoring at duplicate bridge are IMP and MP. IMP are occasionally called International Match Points (like in New York Times articles), but commonly just called imps (like the demonic imp). MP are matchpoints (and that full word is commonly used). So IMPs and matchpoint.

In matchpoint your score is compared to everyone else who held your hand and you either win, tie, or lose depending if there is any difference. In the US that maps to 1 for a win, 0.5 for a tie, and 0 for a loss. In some other places they double those to not deal with fractions. So if there was a matchpoint section with 12 tables and each table played each hand then if you had the very best score (with no ties) you'd get an 11 (and we'd call that a top) since you beat each of the other 11 pairs. If everyone got the same score (like they were all +430 for 3nt making 10 tricks not vulnerable) then everyone would get the same 5.5 score (because you have 11 scores of 0.5) for that hand.

In IMP your score is compared to everyone else who held your hand, but now the difference between your score is what matters. If some people just got 30 points more than you, then that is a small deal, and they'll be +1 IMP against you, and you'll be -1 IMP. If instead they beat you by 530 that maps to 11 IMP. in IMP in fact if the difference between your score is only 10 points (like 4M making 10 tricks and 3nt making 10 tricks) then there is no IMP difference at all.

Consider the following 6 table score for a board:

pair 1: 4♠+1 for +650
pair 2: 3nt+1 for +630
pair 3: 4♠= for +620
pair 4: 4♠= for +620
pair 5: 3♠+1 for +170
pair 6: 6♠X-2 for -500

If you are doing MP then:
pair 1 gets a 5
pair 2 gets a 4
pair 3 and 4 get a 2.5
pair 5 gets a 1
pair 6 gets a 0 (also called a bottom).

If you are doing IMP (and here I'll do cross IMP) then:
pair 1 wins 1 imp from 2, 3, 4 as well as 10 imp from pair 5 and 15 imp from pair 6 for a total of 28 IMP (or +5.6 IMP/comparison)
pair 2 loses 1 imp from 1, gets 0 from 3 and 4, wins 10 imps from pair 5 and 15 from pair 6 for a total of 24 IMP (or +4.8 IMP/comparison)
pair 3 and 4 actually have the same as pair 2, they lose 1 from 1, tie each other an pair 2, win 10 imps from pair 5 and 15 from pair 6 for a total of 24 IMP (or +4.8 IMP/comparison)
pair 5 loses 10 to 1, 2, 3, and 4, but wins 12 from pair 6 so that's -28 IMP (or -5.6 IMP/comparison)
pair 6 loses 15 to 1, 2, 3, and 4, and loses 12 to pair 5 so that's -72 IMP (or -14.4 IMP/comparison)

You can see the imp table here. But notice from the example that in matchpoint the difference in score between pair 6 and 5 was the same (1 matchpoint) as the difference between 1 and 2. In IMP, because the pair 6 had such a bad result, that the difference between 5 and 6 was huge, about the same as the difference between 4 and 5, but the top 4 pairs got mostly the same score. And also notice things like the overtrick went away in importance compared to the pairs that missed game or got set because the IMP table compresses differences the further away from 0 you are.

The above leads to a strategy difference where in IMP scoring it is really important to stretch to bid your games (even ones that are below 50%), and to play to safely make your contract (or on defense set the contract), much more than worry about extra overtricks as declarer or extra undertricks when defending. In MP however, at the highest level approximation, you generally go all out and risking your contract for an overtrick is fairly often the right thing to do (so long as the line you take is more than 50% to work).

In the ACBL IMP are normally used for team games (KO and swiss) and MP are normally used for pair games, so sometimes people will call IMP "team scoring" and MP "pairs scoring". But this is inaccurate as there are MP teams (the BAM or board-a-match) and there are also IMP pairs.