As a novice, we learn that we can preempt at the 2-level with a hand with 6-10 points and six card suit, at the 3-level with the same range but 7 cards, the 4-level with 8 cards and that we might open at the 5-level in a minor suit with either 9 cards or 8 and a side suit, but that generally we don't open 5♥ or higher. Maybe this advice was accompanied by the 'rule of 2 and 3 and 4', which states that if partner is broke you should be within 2 tricks of your contract when unfavourable, within 3 tricks at neutral vulnerability (so all or nobody vulnerable) and within 4 tricks when favourable. Larry Cohen gives some examples of this rule and its application. To my knowledge, bridge players have long joked that the rule has practically been replaced by the 'rule of 3 and 4 and 5' - the same rule, but bid one more. There are some variations ('rule of 2 and 3' - only distinguishing between vulnerable and not vulnerable, not taking the opponents' vulnerability into account), and just the other day there was a Bridgewinners thread joking about the 'rule of 4 and 7 1/2'.
Personally I've learned more about preempts than I thought was ever possible from two concise sources - Andrew Gumperz' sequence on preempts (I only linked the first post but it is a 9-part series) and Kit Woolsey's 14-page chapter on preempts in his 1982 book 'Matchpoints'.
Focusing on the latter for the moment, Kit describes an approach to preempting where there are no hard rules to his bidding, just pros and cons on any given hand compared to what might be expected of the bid. He lists three example 'dreaded' sequences: 3♥-(X)-P-(P); P, 3♥-(P)-P-(P) and the killer 3♥-(P)-P-(X); P-(P)-P (ouch). The chapter then argues that on practically all other continuations the preempt will have gained or not be very costly, so as long as you can avoid these three continuations you should probably preempt (note that near-identical considerations apply for 2- and 4-level preempts). He then lists a number of factors that might influence the likelihood of the auction continuing these ways: internal trump strength (presence of queen through nine), holding the ace of trumps, high cards outside your suit, how suitable the hand is in other strains, vulnerability, position and exchange of information.
Now jumping to Gumperz' series, I think it is sensible to add a few more 'losing' auctions to Kit's list. I suggest 3♥-(3♠)-4♥-(4♠); P-(P)-(pause)5♥/X-(the rest doesn't matter), 3♦-(P)-3NT-(P); P-(P) and 2♥-(P)-2NT*-(P); 3♥-(P)-P-(P).
The point on the first auction is that partner's save may well be a phantom sacrifice, or partner's double may well be assuming defensive tricks in your hand that you don't have. Traditionally suitability for a preempt focuses on offensive values (to make sure you don't go off 4 doubled, or the likes). However, I can't remember the last time I went for -1100, and believe me when I say that I have been trying. What happens far more frequently is that skilled opponents will stretch just a little bit to get their suits in, and partner has to decide whether to play or defend at an uncomfortably high level. It is important to have solid expectations on how many defensive tricks your preempt is expected to contribute.
The point on the second auction is that this is traditionally a very sound auction - if you can be relied on to have something like ♠Kx, ♥x, ♦KQTxxxx, ♣Jxx partner will probably claim 9 or 10 tricks shortly. However, if you wait for a hand like that to open 3♦, you are leaving serious IMPs on the table. The main message a preempt should convey is 'my hand is suitable for this strain, but very poor on support or defensive values in other strains'. With this style partner needs a powerhouse of a hand, with special emphasis on good support in our suit, to play in 3NT. A balanced 17-count with Ax in support doesn't cut it. To be clear - I'm not saying you have to copy this style, but that you have to choose between either preempting (far) less often than I think is wise, or bid impossible 3NT with some frequency, or pass out some strong hands over partner's preempts. In my opinion this is a discussion worth having in a committed partnership.
The last auction is about playing strength. If you go too far and your preempts promise absolutely nothing, partner will regularly have to investigate game and then pull the brakes one level higher than intended. The Bergen & Cohen partnership was famous for this, to the point they adopted 2-under transfers at the 2-level to distinguish the utter garbage from the normal preempts (next step asks, and garbage signs off). Similarly to the previous example auction, you have to find a balance between the frequency of your preempts, the type of hands that can make game tries and your willingness to be one level higher than the field.
Based on all the above I'd add some factors to Kit's list for evaluating preempts, arguably with some overlap: trump length, HCP, side voids, offence-to-defence ratio. This makes my list, sorted by importance:
- Position
- Vulnerability
- Exchange of information
- Offence-to-defence ratio
- Trump length
- Internal trump strength
- Suitability for other strains
- Side voids
- Ace of trumps
- HCP
This is probably a lot to take in, so let me go through a few examples.
mikeh, on 2022-March-07, 13:04, said:
- Position: +, first seat is the second best position (3rd-1st-2nd) for preempting.
- Vulnerability: ++, white versus red is the best (white v red - white v white - red v red - red v white).
- Exchange of information: ++, the opponents have no idea what's going on.
- Offence-to-defence ratio: ++, the queen of clubs might come home on defence but I'm not holding my breath. That ten of diamonds is a great asset on offence and worthless on defence.
- Trump length: -, nominally partner would expect a 7-card suit for a 3-level preempt (well, in the old days).
- Internal trump strength: +, we're missing jack and nine.
- Suitability for other strains: +, we might belong in clubs but at least we don't have anything in the majors.
- Side voids: +, no side voids.
- Ace of trumps: +, we don't have it.
- HCP: +, 7 points is in the normal range.
♠x, ♥AJTxxxx, ♦Kx, ♣xxx (from Gumperz' article), second hand all red.
- Position: -, second seat is the worst position.
- Vulnerability: -, all red is the second worst vulnerability.
- Exchange of information: -, the pass to your right limits the opponents' prospects, while partner's holding is unknown.
- Offence-to-defence ratio: -, your ace of hearts always rates to take a trick, and the diamond king is about as useful on defence as it is on offence.
- Trump length: +, you have seven of them.
- Internal trump strength: +, we're missing queen and nine but that's reasonable.
- Suitability for other strains: ++, this hand only plays in hearts (unless partner insists on NT).
- Side voids: +, no side voids.
- Ace of trumps: -, we have it.
- HCP: +, 8 points is in the normal range.
♠KQT9x, ♥x, ♦ATxx, ♣Jxx, white versus red, third seat. Consider 2♠.
- Position: ++, third is the best position.
- Vulnerability: ++, we are white versus red.
- Exchange of information: +, the pass to your right limits the opponents' prospects, but partner's pass practically rules out that we have a game.
- Offence-to-defence ratio: +, the ace of diamonds is a bummer but the heart shortness suggests taking our their likely heart contract anyway, and the ten of spades is a good offensive card.
- Trump length: -, we only have five instead of the normal six.
- Internal trump strength: ++, we're just a jack short of a long sequence. As an aside: there is next to no chance that we get doubled, because the opponents are lacking the spot cards to leave it in.
- Suitability for other strains: --, we might well belong in a minor suit.
- Side voids: +, no side voids.
- Ace of trumps: +, we don't have it.
- HCP: --, This hand is well over strength for a preempt.