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© boco_san, 2012/August

Preface

Conclusion

Results Obtained

Appendix: How to Calculate the Probabilities

Appendix: When Has the Transition Occurred

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      Once an opening 1NT bid used to mean balanced hands with  (B) 16 -18 HCP .   Meanwhile, perhaps in 1980's, the HCP range has changed to  (A) 15 -17 HCP .  
      It is obvious that the choice (A) will bring more chance of 1NT opening, and will allow easier bidding sequence that will follow.  
      Surely often, but, how often ?   Often to what extent ?   In other words, how more often will you pick up 15 -17 balanced hands than 16 -18 ones ?  
      I know no answers in Bridge books nor in any WEB sites.   So, I have tried for myself to evaluate probabilities of the (A) and (B) hands with balanced distributions, [4333], [4432], or [5332].   This distinction of balanced hands is so essential, because we never open 1NT on hands with a singleton or a void.
      I'll start with the conclusion and then explain outline of the calculation.   Details are given in the appendix below.

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      For the sake of hasty Bridge players, I will give first the conclusion.  
When we compare the relative frequency of the two kind of hands,
         (A) 15 -17 HCP  balanced hands,
         (B) 16 -18 HCP  balanced hands,
the former will appear 1.371 times more frequently than the latter.
      It is as well possible to do this comparison for
         (a) 15 -17 HCP  balanced as well as unbalanced hands,
         (b) 16 -18 HCP  balanced as well as unbalanced hands.
Although this comparison is totally meaningless, relative frequency of (a) to (b) turns out to be   1.387.   This result is known in the WEB as 38%.   So, I learned that the restriction to balanced hands was not important.
         If you are interested in details, read more.   Actually, it was a hard task for me.

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      The problem here is to evaluate exactly the probabilities of picking up (A) 15 -17 and (B) 16 -18 HCP balanced hands.
      When a player is dealt 13 cards, the number of card combinations amounts to ₅₂C₁₃ = 635,013,559,600 (about 635 billions).   One can classify these deals according to HCP in the range 0 up to maximum 37.   This doesn't solve our problem, however.
      1NT opening requires balanced distributions in the four suits.   Actually, only the three kinds of balanced distributions [4333], [4432] and [5332] will occupy 47.6% out of the 635 billion deals (which is somewhat astonishing in view of the possible 39 kinds of distributions).   So, we encounter balanced hands almost once in two deals, which implies importance of how to bid on balanced hands (with only 3 kinds of distribution).
      For the purpose mentioned above, we now classify the 635 billion deals according to both distributions and HCPs.   The result is given in the following two big tables:
            (1) Number of Hands
            (2) Probability (in units of ‰ = 1/1000)
Click either table to see.
      Both tables consist of  38 rows (for 0-37 HCP) × 39 columns (distributions) = 1,482 cells.
      Table (1) gives the number of deals for specified HCPs AND distributions.   They will sum up in total to 635 billion, as shown in the right bottom cell.
      By simply dividing these numbers by 635 billion, we get the probabilities as shown in Table (2).   Here, blanks mean that the corresponding probability is absolutely zero (and hence they never occur), while “0” means non-vanishing probability smaller than 0.001%.   Because of the very small probabilities owing to the fine division into 1,482 cells, the probability values are shown in units of ‰ rather than %, and hence their total becomes 1000, again shown in the right bottom cell.

HCP	[4333]	[4432]	[5332]	Total	%
15	0.469	0.958	0.688	2.115	(A)  15 -17  Total 4.866
16	0.355	0.723	0.182	1.596
17	0.259	0.523	0.373	1.155	(B)  16 -18  Total 3.550
18	0.180	0.363	0.256	0.799

      Now, we extract the blue sections of Table (2) corresponding to balanced hands and 15 -18 HCP, to obtain the table shown on the right.   Here, we restored the units %.
      This extracted table tells us that (A) and (B) will occur with absolute probabilities 4.87% and 3.55%, respectively, and hence we find the ratio of frequency 4.866 / 3.550 = 1.371.
      To conclude, (A) will allow 37.1% more often 1NT opening than (B).   Plainly speaking, while players (A) calls 4 times 1NT, players (B) will call only 3 times 1NT.  This marked difference arises owing to the difference of probabilities of 15 and 18 HCP hands.
      The situation may be understood clearly from the figure on the right.   The curves (circles) stand for probabilities against HCP taken in the abscissa.
      As mentioned above, the black circles (all hands, both balanced and unbalanced) show almost twice as large probabilities as the blue ones (balanced hands only).   Both curves have a peak around HCP=10, since 40 HCPs are distributed among four players.   Because the range 15 -18 is situated at the (right-hand) foot of this peak, the probability will increase considerably when one discards 18 HCP and takes 15 HCP.

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