[lycier]

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I think 3♣ as a Bar bid only with 5 cards is very unreasonable because the purpose of our bidding is to look for 8-card suit fit,if it have only 5 card,the contract with 5-2 fit is very in danger.
For this hand,Gib W pass is better instead of 3C-bar bid after 1nt.
So I think the bar bid with 4-6 distribution is very reasonable.

[1eyedjack]

What is the probability that a 1N rebid by East has fewer than 3 Clubs, having already shown fewer than 4 Spades, fewer than 4 Hearts and fewer than 6 Diamonds? ie precisely 3-3-5-2 shape?

I don't know the answer. Should be trivial to find out, but instinctively I would put it at the low side, sufficient to justify the gamble, if an 8 card fit were sufficient to justify the bid.

That said, if you ONLY have an 8 card Club fit, the merits of playing in 3C contrasted with 1N when West also has a fundamentally balanced hand are questionable, so that while I am unconvinced by the logic presented above, I tend to agree with the conclusion.

[helene_t]

Does GIB do simulations here to determine if 3♣ is better than 1NT? The hand is strong enough not to worry about 1NT. So I would expect a sim to favour 1NT. But I could be wrong, or it could be a statisticial fluke.

[lycier]

That is a possible speculation.
With a misfit bid cautiously,with a good fit be bold.

[uva72uva72]

The problem with this, as with other, "retreat from no trump" sequences is that the range extends much too high. With 11 hcp, responder, in my view, should be closer to inviting game than to running from no trump. At matchpoints, with as few as 9hcp I'd be looking to play no trump rather than run, unless my minor was long (6+ cards) and weak. Another problem is that the "5+" minor stipulation can result in jumps to the 3 level on really bad suits - I've seen Jxxxx. In my experience, this inevitably results in the worst of all possible worlds - playing 3 of a minor vice 1NT with 23-25 hcp and a trump suit of Ax opposite Jxxxx, down 1 vulnerable, losing to all those who passed out the hand or who sold out to 1 of a major for -80. It's also common to see North run out in the sequence 1♦-1♥;1nt-2♦ with 11 hcp although this is again described in the notes as a "retreat from no trump." This has caused some savvy Souths to retreat to 2nt from North's retreat from 1nt. Is it not possible to narrow the range for these bids and increase the suit length requirement for jumps to the 3 level?