When deciding if we should bid game or slam in imp play it is important to understand what the gain will be if we make the game or slam and what the lose will be if we fail,

We have four situations:
1. Bidding a Vulnerable Game.  If we play a vulnerable 4 and make our contract we will score 620 instead of 170. The difference between these scores is 450.  If you look at the bottom of the inside of a convention card you will find the International Matchpoint Scale. A 450 point swing is worth 10 IMPs. If we do down one in 4 we would lose 170 and 100.  We would lose 270 points or 7 IMPs. This means we should be very aggressive in our vulnerable game bidding. If we have a 42% chance of making the game we should bid it.

2.  Bidding a Non-Vulnerable Game.  If we play a non-vulnerable 4 we gain 6 IMPs by making it and lose 5 IMPs if we fail. I am not as aggressive when bidding IMP non-vulnerable games than when biddint IMP vulnerable game but I am more aggressive than I am in match point.

3.   Small Slams.  We gain or lose about the same when we make or fail in both vulnerable and non-vulnerable small slams. I usually need about a 60% or better chance to make may small slams. 51% small slams should be bid in IMP scoring.

4.  Grand Slams.  We gain 750 points (13 IMPs) for making a vulnerable grand slam;  We lose about 1400 (16 IMPs) when we go down  We gain 500 points (11 IMPs) and lose 1000 points (14 IMPs) if we are not vulnerable .  I try to count 13 tricks before bidding grand slam in both match point and imp scoring.