Against a trump contract you lead the A in your long suit not having the K, but hoping to find partner with shortness. You are partially right, since your partner and declarer share 3 small between them, and they are either 2-1 or 1-2 (you get the trick!). Would you now prefer to play Standard counts or UDCA?
Say you are missing the T, 6 and 2. Then the following scenarios are relevant, if your agreement is Standard counts:
* If partner plays the 2, you know he has a singleton no matter what declarer plays.
* If partner plays the 6 (from 62 or 6), you know he has the singleton, if declarer plays the 2, and a well-prepared expert declarer will therefore always(!) play the T, thereby making your guess 50-50.
* If partner plays the T (from T6, T2, or T), your guess of him having a doubleton will be correct in 2/3 of the cases, no matter what declarer plays.
The scenarios are equal, if your agreement is UDCA. However, if your partner now plays the 6 (from T6 or 6), the well-prepared expert declarer must always play the 2 in order to make your guess 50-50.
It is my contention that in this example, the non-expert (or the unprepared expert) is more likely to play the same card as the well-prepared expert, if you play UDCA, than if you play Standard counts, simply because it is more frequent to play the 2 than the T from T2, when you do not know that this is a crucial choice. Of course, if declarer erroneously think he should play randomly in this situation, he will choose the wrong card ½ of the times, no matter whether you play Standard counts or UDCA, but I propose that it is more frequent for this declarer to play the 2 from T2 than the T. Do you agree?
I am not skipping UDCA just because of this example, but I am a little shaken. Anyone with examples that favor UDCA?
/Niels
PS: I think there is a rule for expert declarers to use the same signals as the opponents. That is: play your cards, as if you play their counts or their encouragements. In the situation described above: As declarer, you should play the T from T2, if it shows doubleton in opponents agreements, but 2 from T2, if their agrement is UDCA. If you have heard about that rule of thumb, too - do you have a reference?
PPS: In the above example, all statistics were a priori. When the guess is "purely" 50-50 (partner plays the 6, and declarer plays the right card), I believe it is probably more likely that partner has the doubleton, since declarer will typically have more trumps, and therefore less free space, than partner. Of course, if partner plays his card very fast, the situation is quite different...!
