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When do you bid a Grand Slam at MPs and IMPs?

The bidding has reached six of a minor suit, and you must make a decision for the final contract based on what you know.

You do not know and can not determine whether partner has three card or four card support in the minor suit.  If partner has three card support, then grand slam in the minor will make 40% of the time.  If partner has four card support, then grand slam in the minor will make 80% of the time. 

Given:

-- The probability of partner having three card support is 50%.

-- You are considering NT as a final contract.  To make the same number of tricks in NT as in the minor suit, a finesse must be on.  No finesse is needed in the minor suit contract.  If the finesse loses, you lose one more trick in NT than you would in the minor suit contract.

-- If partner has three card support, then the probability of a small minor suit slam making is 80%.

Summarizing the probability table (from the above):

1. Probability of grand slam in the minor suit contract making = 60%

2. Probability of grand slam in NT making = 30% (Half of "1.")

3. Probability of small slam in the minor suit contract making = 90%

4. Probability of small slam in NT making = 75% ("1." + half of "3. minus 1.")

What is your decision on where to place the final contract, at IMPs and at Matchpoints, assuming that you are playing the first board in a top flight event?

Comments are welcome about how your decision would be influenced by the probability of "1." above.  Assume you are non-vulnerable, if that makes a difference (does it?).

Small Slam Minor at both MPs and IMPs
Small Slam Minor at IMPs, but 6NT at MPs
Grand Slam Minor at both MPs and IMPs
Grand Slam Minor at IMPs, but 6NT at MPs
Grand Slam Minor at MPs, but Small Slam Minor at IMPs
None of the Above (Please Explain)

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