[iviehoff]

1. There's two issues (a) what West already knows, which we don't have enough info about the hand to know and (b) whether knowing it he would in practice get the discard right. I think we would need to test this by organising a poll. Give people the bidding and play to date, assume that South wins a spade on table, cashes the heart and discards a spade (diamond makes it too easy), see how often polled Wests get the discard right.
2. No. Your question perfectly exposes what seems to be a misunderstanding underlying the devising of this law. The law only makes sense if one presumes the lawmakers assumed that the claimant's partner would be objecting to the loss of specific conceded tricks that he expects to win, and at times that may be the case. But the law means it is often advantageous to you to object to partner's claim simply if think partner's claim is wrong, because usually you can expect to win more tricks playing it out than you will get under an adjudication, since usually the constraints of the UI laws on what you can do will be less constraining than the impact of the "careless" criterion. Why you should be able to take advantage of this precisely when partner claims for less than all of the tricks, but not when he claims for all of the tricks, defies logic. But it is a law so infrequently applied that I doubt the lawmakers can get it high enough up their list of priorities to fix. And then there is the risk they will write something even worse.

[lamford]

iviehoff, on 2011-August-26, 06:43, said:

1. There's two issues (a) what West already knows, which we don't have enough info about the hand to know and (b) whether knowing it he would in practice get the discard right. I think we would need to test this by organising a poll. Give people the bidding and play to date, assume that South wins a spade on table, cashes the heart and discards a spade (diamond makes it too easy), see how often polled Wests get the discard right.

Too elaborate in my view. Fine to poll players to decide what is an LA, but the above is not the way to judge what is careless. The players will be straining to work out declarer's shape, and even if they have a complete count of the hand, they can still be careless. I would say that ALL discards which are plausible are treated as careless. If, however, West had S 432 and D Q3 left, then I would say that it would be irrational to keep a spade which cannot be a guard. But if West had S 532 and D Q3 left, he would be deemed to discard wrongly. If declarer had shown out of a suit, then I would allow the defender to remember that, but I would not accept the argument that he had a "complete count of the hand". To have miscounted is careless. How many times have you kept the thirteenth card in a suit? I certainly have.

[barmar]

I've seen decent players (my partners, unfortunately) discard incorrectly because they didn't notice someone showing out of a suit or that the suit had been played 3 times and their card was the useless 13th.