Drawing the correct conclusion
Benjamin Stolberg, an American journalist who died in 1951, defined an expert as someone who avoids small error as he sweeps on to the grand fallacy. However, in bridge it is important to know whether your opponents are experts or less capable practitioners. You can construe credible conclusions from the play of the former. The deductions are less dependable from the latter. South didn't find the winning line on today's deal, which he played against two international champions. Was he unlucky, or did he miss a key piece of evidence?
West led the spade six, declarer correctly ducking the first trick. Back came a spade to dummy's king. Declarer saw that if the spades were 5-3, as they appeared to be, he would be defeated if West had both missing aces. If East had them both, South was safe, but if the aces were split, as they were likely to be, he had to drive out West's ace first: West's entry to his long spades.
South played a diamond to his king, which West ducked. Deciding this meant East had the diamond ace, South now led a club to dummy's queen. However, East won with the ace and returned his last spade. When declarer played another diamond, West won with the ace and cashed two spade tricks: down one.
West played well in ducking his ace. He could see that although he might lose a diamond trick, he had to retain his entry card. However, South shouldn't have assumed East held the diamond ace, because if East did have the ace, he would have played it immediately at trick three. He would have identified the importance of preserving his partner's entry.