The difference between MatchPoints and IMPs is simple enough. In MatchPoints, your score is put on a list with everyone else's score. You get one MatchPoint for every person you beat on this list, a half a MatchPoint for everyone you TIE on that list. Hence, if you do better than the 12 other people who played the hand, you would get 12 MatchPoints. An average result would be 6 MPs if 13 pairs play the hand. On OKBridge, this is simply converted to a percentage with 100% going to anyone who beats all of the other pairs, 0% to anyone who registers the worst score.
In International Match Points ("IMPs"), we take the DIFFERENCE between the scores that you and your opponents have achieved and convert that difference to an arbitrary number of "IMPs" according to this scale:
Difference in Raw Scores IMPs 0-10 0 20-40 1 50-80 2 90-120 3 130-160 4 170-210 5 220-260 6 270-310 7 320-360 8 370-420 9 430-490 10 500-590 11 600-740 12 750-890 13 900-1090 14 1100-1290 15 1300-1490 16 1500-1740 17 1750-1990 18 2000-2240 19 2250-2490 20 2500-2990 21 3000-3490 22 3500-3990 23 4000+ 24
IMPs were originally designed for team competition. IMP PAIRS is simply an averaging of your opponents' results before such a comparison takes place; you are pitted against the "average opponent" when playing IMP Pairs.
Playing IMPs, if you make 430 while others make 420, you have a raw score difference of a measly 10. This is worth ZERO IMPs. Were you playing MatchPoints, though, 430 would beat ALL the other pairs with their 420 scores; you would receive ALL the MatchPoints on such a board. (This, by the way, explains the popularity of No Trump contracts among MatchPoint players.)
Similarly, if EVERYONE ELSE scores +100 on a hand, you will get all the MatchPoints with any score above +100: +110 would get you the same fine result as +1100 would!
At MatchPoints, then, you only need to beat the score achieved by others who play the same hand. It does not matter BY HOW MUCH you beat them.
At IMPs, though, it is not enough to beat the others who hold the same hand as you. In order to make headway, you must beat them by as much as possible! If you score +110 as opposed to +100 you will score a lot of MatchPoints, but a net difference of (110 - 100 = ) 10 would net you ZERO IMPs. +1100 versus everyone else's +100, on the other hand, is worth (1100 - 100 = ) 1000 net points, or a whopping +14 IMPs.
How does this difference in scoring methods affect your decisions at the table? Plenty!
At MatchPoints, making an overtrick in an undoubled, non-vulnerable 4H while others make only ten tricks means a "top" board (i.e. all the MatchPoints available). But at IMPs, it means only (450 - 420 = ) 30 points, or 1 measly IMP! With so few IMPs at stake, this would be considered a "flat" or "near-flat" board at IMPs. Many such hands which are so exciting at MatchPoints ("Can you make that critical extra trick?") are a total yawn at IMPs!
On the flip side, going down in 4H while all others make 10 tricks is a "bottom" board, or ZERO MatchPoints. At IMPs, though, you have just lost (420 + 50 = ) 470 points in total, which converts to a disastrous -10 IMPs.
From this we discern that, at IMPs, you would NEVER risk the contract for an overtrick. The risk (-10 IMPs here) far outweighs the gain (1 IMP). At MATCHPOINTS, however, that overtrick may mean the difference between a top and an so-so result, so you MIGHT, if you see the chances as better than 50%, put your contract at risk to try for an overtrick (especially if you think everyone will be in the same contract as you). Similarly, on defence, you would always cash the setting trick at IMPs to ensure defeating the contract. But, at MatchPoints, an extra undertrick might be worth the risk of NOT cashing the setting trick in the hopes of setting the contract TWO tricks!
Making Up For A Bad Board
"So," you might ask, "if going down in a game which everyone else makes is a disaster at both forms of the game, what's the difference? Isn't it just one bad board?"
The difference is that at MatchPoints you can recover by making an overtrick in 3D on the very next board while everyone else makes only 9 tricks. At IMPs, this would only recover ONE of the ten IMPs you lost going down in 4H earlier! Because you need hands where big numbers are at stake, it is much harder to recover from disasters at IMPs BECAUSE THERE ARE FAR MORE "FLAT" OR "NEAR-FLAT" BOARDS AT IMPs!
In MatchPoints, "the play is the thing". An overtrick or an extra undertrick means EVERYTHING if it distinguishes your score from everyone else's. There are few "flat" boards at MatchPoints. In a typical 24-board session at MatchPoints, you may have 2 or 3 "flat" boards. In a typical 24-board match at IMPs, you may have 12-18 such "flat" or "push" boards. :(
At IMPs, bidding is paramount. Miss a slam or a makeable game and you're in trouble. Squeezing out an extra overtrick or extra undoubled undertrick via careful play on the next hand will NOT recover your loss.
The above distinction explains the expression: "IMPs is for BIDDERS; MPs is for CARD-PLAYERS!" It also explains why MatchPoints is a much more intense game where EVERY board has equal importance. A delicately bid and played 7NT contract will not gain you any more or any fewer MatchPoints than the lucky overtrick you made in 1C the hand earlier. At IMPs, however, you can safely fall asleep during the 1C hand--as long as you wake up for the 7NT one! In this regard, by the way, IMPs is closer to Rubber bridge than MatchPoints is.
Bottom line: At IMPs, the big "swing" hands with a lot of points at stake (e.g. difficult games, slams and doubled contracts) matter most. At MatchPoints, all boards are equally important.
Part Score Bidding
The area where the two games diverge most is in competitive part score bidding. Consider this situation: You have bid 3D, rather confident of making. The vulnerable opponents compete to 3S. You can't be 100% certain, but your instincts tell you that 3S will likely go down one. Should you double?
At IMPs, the difference between 3S down one doubled versus undoubled is (200 - 100 = ) 100, or THREE IMPs. If they make, though, the difference is (730 - 140 = ) 590, or ELEVEN IMPs. Hence, you'd better be VERY confident of 3S going down before doubling it at IMPs! Otherwise, you would do better to pass 3S. If it does go down one, the board will be flat, since (110 - 100 = ) 10 points means ZERO IMPs.
At MatchPoints, however, you were slated for +110 in 3D, so +100 in 3S will NOT compensate you. You may get a bottom board if everyone else is allowed to collect +110 in 3D. Hence, to "protect" your +110, you might double 3S and hope to nip it one for +200 and a GREAT score at MPs.
Bottom line: save those close doubles for MatchPoints, not IMPs!
At MatchPoints, you will often strain to balance the enemy into an unmakeable contract. Nothing does your game better than pushing the opponents to 3H and collecting +50 while everyone else is -110 against 2H. If balancing against their 2H risks the odd -800 because of a trump stack against you, so be it. As long as you collect more +50s than -800s you'll do fine in the long run at MatchPoints.
At IMPs, though, those -800s had better be FAR fewer. Getting +50 versus -110 is worth (50 + 110 = 160) FOUR IMPs. -800 versus -110 loses (800 - 110 = 690) TWELVE IMPs, and such disasters are harder to recover from, with fewer decisive boards at IMPs than at MatchPoints. :(
Bottom line: save your balancing heroics and aggressive part score competing for when you're playing MatchPoints, not IMPs!
Let's take another scenario. They have bid 4H, vulnerable. You figure that they have a better-than-even chance of making it. Your side, on the other hand, can bid 4S, get doubled, and go down -500, 120 points better than you will spit up for 4H if it makes. Should you sacrifice in 4S, then?
At MatchPoints, the answer is YES. If they do make 4H you are have improved your score from -620 to -500. Yes, if 4H goes down, you -500 will not look good opposite everyone else's +100. But as long as 4H has a better than 50% chance of making, you will gain more often than you will lose IN THE LONG RUN.
At IMPs, the answer not so clear. 4S will only improve your score by (620 - 500 = 120) THREE IMPs. If, on the other hand, 4H goes down one, you will have foregone the +100 for -500, a 600 point difference for MINUS TWELVE IMPs. Since your sacrifice has to be "correct" (i.e. 4H has to make) 4 times to 1 for those 3 IMPs to equal the 12 IMPs you might be missing, 4H should have an 80% or better chance of making before you should sacrifice against it! At MatchPoints, your sacrifice only had to be right 51% of the time to pay off in the long run!
Bottom Line: At MatchPoints, sacrifice against anything the opponents bid confidently. At IMPs, sacrifice only against those contracts which are underwritten by Lloyds of London! :)
And that, in a nutshell, is the difference between IMPs and MatchPoints.