Principle of Restricted Choice - Example

Example

Consider the suit combination represented in the figure. There are four spade cards ♠8754 in the South (closed hand) and five ♠AJ1096 in the North (dummy, visible to all players). West and East hold the other four spades ♠KQ32 in their two closed hands.

South leads a small spade, West plays the 2 or 3, dummy North plays the ♠J, and East wins with the ♠K. Later South leads another small spade and West follows low again. Now only one spade card is "unknown" to South, not yet observed or inferred, with North and East yet to play on the second spade trick. Is it better to play ♠A, hoping to drop ♠Q from East, or to finesse again with ♠10, hoping to drop ♠Q from West on the third round of the suit? That is, should declarer play for the defenders' original holdings to be 32 and KQ or Q32 and K? The principle of restricted choice explains why the latter is now about twice as likely, so that to finesse again by playing ♠10 is nearly twice as likely to succeed.

Prior to play, 16 possible West and East spade holdings or "lies" are possible from the perspective of South. These are listed at left, ordered first by "split" from equal to unequal numbers of cards, then by West's holding from strongest to weakest.

After West follows to the second spade, which is the moment of decision referred to above, only two of 16 original lies remain possible (bold), for West has played both low cards and East the king. At first glance, it may seem that the odds are now even, 1:1, so that South should expect to do equally well with either of the two possible continuations.

However, this is not the case because if East had ♠KQ, he could equally well have played the queen instead of the king. Thus some deals with original lie 32 and KQ would not reach this stage; they would instead reach the parallel stage with ♠K alone missing, South having observed 32 and Q. In contrast, every deal with original lie Q32 and K would reach this stage, for East played the king perforce (without choice, or by "restricted choice").

If East would win the first trick with the king or queen uniformly at random from ♠KQ, then that original lie 32 and KQ would reach this stage half the time and would take the other fork in the road half the time. Thus on the actual sequence of play, the odds are not even but one-half to one, or 1:2. East would retain queen from original ♠KQ about one-third of the time and retain no spades from original ♠K about two-thirds of the time.

Importantly, this assumes that the defenders have no signalling system, so that the play by west of (say) the 3 followed by the 2 does not signal a doubleton. During the course of many equivalent deals, East with ♠KQ should in theory win the first trick with the king or queen uniformly at random; that is, half each without any pattern.