O/D Parity and Total Tricks
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When talking about total tricks most players would automatically think of LOTT - Law of Total Tricks. It said the total tricks for a given hand is equal to the sum of number of trumps (best suit of each side) from two sides. However it is well know in expert players that this Law breaks down at higher level of bidding.

Here I present a different way to estimate Total Tricks for a hand. Let's assume your side could take X tricks in offense, but only Y tricks if playing defense. The opponents would be able to make 13-Y tricks if they play the hand. By definition the total tricks for this hand is X + (13 - Y).

I could rewrite this formula to (X-Y) + 13.

What is (X-Y)? It is simply the difference of number of tricks you could take in offense vs. defense (Offense/Defense Parity). So you don't need to know the exact number of X and Y, only the difference matters.

For a lot of hand, certain cards could win a trick in either offense or defense and they DO NOT change the number of total tricks. In a similar way a card (like a K) might be a loser if opponent happen to have A behind it. It is not a winner in either offense or defense and it does not change number of total tricks.

What determine the number of total tricks? Let's look at the following deal from a recent BBO game.

West
95
K10743
KQ76
107
North
K832
5
A93
AK954
East
QJ106
AQ862
J84
Q
South
A74
J9
1052
J8632
W
N
E
S
P
1
X
3
3
4
?
D

For this hand each side has 10 trumps. LOTT says there should be 20 total tricks. However neither side could make anything at 4 level. There is only 18 total tricks.

Now let's look at NS hand, the bidding showing NS has 10 cards in  and EW is known to have a  fit. If  is split 2-1, NS would make 4 more tricks from suit in offense than defense. North has a singleton in . If EW has 10 cards in , north could also make an extra trick by ruffing . This brings X-Y to 5 for NS. 5+13 = 18 total tricks. EW could go through same exercise. Notice the high card in  and  (side suit) has no effect to total tricks in this hand.

In real game EW bid 4 on this hand and North compete to 5, not the best decision.

Other factors that determine X-Y are the locations of QJ and distribution of side suits. A typical case is one player has Qxx in opponent suit and partner has Jx. This is 1 trick in defense but 0 trick in offense, thus reduces total tricks. Distribution of side suit like a second suit fit may increase X-Y and increase total tricks.