Forcing NT vs Non-Forcing NT
A common criticism of two over one game force is the necessity for some level of forcing NT. In recent years, many expert-level players have adopted a so-called "semi-forcing" NT, which is really just non-forcing. Nevertheless, the forcing NT does still remain popular. I decided that I would try to quantify the cost and benefit of either choice.
To do this, I generated 20,000 hands, using the Bridge Lab feature on Bridgetricks, 10,000 for a 1S opening and another 10,000 for the 1H opening. The spade hands were constrained to have 5-7 spades, and could have 0-5 in every other suit, with an HCP count in the 12-14 range. The heart hands were constrained to 0-4 spades, 5-7 hearts and up to 6 in either minor.
The paired responder hands were constrained to 0-2 spades, and up to 6 in any other suit for the spade opener, and 0-3 spades, 0-2 hearts and up to 6 in the minors for the heart opener, both in the 6-9 HCP range.
I didn't constrain this further, so that I could implicitly account for the frequency of hands where forcing and non-forcing NT might behave differently.
I also needed to define continuation rules. Given that the hands are quite constrained, this was relatively straightforward.
- Non-forcing: only rebid with a 6+ card major or 4-card side suit below the 2-level of the opening suit, otherwise pass. Responder then chooses between the two alternative, or bids a new 6-card suit at the 2-level. If opener repeats his suit, responder always passes.
- For forcing continuations, I tested two main ideas: with no natural rebid, opener either bids his lowest 3-card minor (better minor), or rebids clubs with as few as 2 cards (short club). Responder then passes if he can guarantee 7 cards in the suit, i.e., if opener only promises 2 clubs with his 2C bid, then responder only passes when holding 5+ clubs.
- I then tested two different variations of this, where responder either deferred to opener's major with 2 cards with top priority, or bid a new 5-card suit with priority. New 6-card suits were always bid with higher priority than deferring to opener's first major.
- Finally, I also tested the BART convention, which is normally used to show invitational values, just to to see the cost.
Below, please find a pivot table of the result from the 20,000 boards. In this simulation, the "Oracle" is simply what a double dummy solver gives as the best contract. Where two contracts give the same score, the listed contract is the higher ranked one, prioritising North over South.
The headline result is that on hands where opener bids a major, and responder bids 1NT, the best forcing method simulated gained 7 points per board over the non-forcing NT, or about 0.35 IMPs. Where opener is very weak, this gain increases to 9 points per board.
On boards where 1NT is genuinely the best contract, according to DDS, the non-forcing NT outperforms forcing variants by 25-35 points per board, but these boards are not common enough to make up the difference where a suit contract is better.
Also of interest, is that bidding clubs with as few as 2 clubs generally outperforms bidding the better minor by quite a bit, but gives back a lot of that gain on boards where opener has genuine clubs, as responder cannot pass with less than 5 clubs. However, allowing responder to pass with 4 where the clubs could be 2 resulted in a worse result.
There also appears to be a very marginal gain for bidding 2 of opener's major ahead of bidding a new 5-card suit, but it is possible that this is within the margin of error.
The result was quite surprising to me, as normally the argument is that forcing 1NT creates a cost. If anyone wants, feel free to DM me, and I can send you my excel or the PBNs I used.