# BBO Discussion Forums: Why this "Rule of Thumb"? - BBO Discussion Forums

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## Why this "Rule of Thumb"?

### #1Wentys

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• Joined: 2023-February-17

Posted 2024-March-19, 22:48

In the GIB article that explains how the Robot describes its bids, they say "This takes into account the form of scoring; this is how we emulate rules of thumb like "bid games more aggressively when vulnerable at IMPs". https://www.bridgeba...escriptions.php

Can someone explain the justification for this rule of thumb, and whether it is widely agreed or used by more advanced bridge players?

Thanks
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### #2smerriman

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Posted 2024-March-19, 23:25

Suppose you're choosing between playing 3M and 4M.

Non-vulnerable, if you bid and make game you gain 6 IMPs (+420 vs +170), while if you go down 1 you lose 5 IMPs (-50 vs +140). So you want to bid game if it's making at least 5/11 = 45.5% of the time.

Vulnerable, you gain 10 IMPs if you make game (+620 vs +170), while if you go down you lose 6 IMPs (-100 vs +140). So you want to bid game if it's making at least 6/16 = 37.5% of the time.

There's a small chance both are down, but that doesn't impact the results much.

Of course, distinguishing between 37.5% games and 45.5% games may not be easy to do at the decision point, but it just goes to show how low a chance a game needs to be vulnerable to be worth bidding it.
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### #3Wentys

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Posted 2024-March-20, 02:43

smerriman, on 2024-March-19, 23:25, said:

Suppose you're choosing between playing 3M and 4M.

Non-vulnerable, if you bid and make game you gain 6 IMPs (+420 vs +170), while if you go down 1 you lose 5 IMPs (-50 vs +140). So you want to bid game if it's making at least 5/11 = 45.5% of the time.

Vulnerable, you gain 10 IMPs if you make game (+620 vs +170), while if you go down you lose 6 IMPs (-100 vs +140). So you want to bid game if it's making at least 6/16 = 37.5% of the time.

There's a small chance both are down, but that doesn't impact the results much.

Of course, distinguishing between 37.5% games and 45.5% games may not be easy to do at the decision point, but it just goes to show how low a chance a game needs to be vulnerable to be worth bidding it.

Thanks, just what I was looking for!
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### #4bluenikki

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• Joined: 2019-October-14

Posted 2024-March-24, 20:47

smerriman, on 2024-March-19, 23:25, said:

Suppose you're choosing between playing 3M and 4M.

Non-vulnerable, if you bid and make game you gain 6 IMPs (+420 vs +170), while if you go down 1 you lose 5 IMPs (-50 vs +140). So you want to bid game if it's making at least 5/11 = 45.5% of the time.

Vulnerable, you gain 10 IMPs if you make game (+620 vs +170), while if you go down you lose 6 IMPs (-100 vs +140). So you want to bid game if it's making at least 6/16 = 37.5% of the time.

There's a small chance both are down, but that doesn't impact the results much.

Of course, distinguishing between 37.5% games and 45.5% games may not be easy to do at the decision point, but it just goes to show how low a chance a game needs to be vulnerable to be worth bidding it.

But of course these numbers assume you will not be doubled.
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### #5mycroft

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Posted 2024-March-25, 13:49

Another one of those rules is "never double without a trump stack", and "never double for a 1-trick set".

Because:
NV: 590 vs 170: 9 IMPs. -100 vs +140: 6 IMPs. 6/15 = 40%.
V: 790 vs 170: 12 IMPs. -200 vs +140: 8 IMPs. 8/20 = 40%.

I think this chance (plus smerriman's "hard to tell the difference") is why the common guidance is "bid 50% games NV, 40% V". Puts a little EV into your pocket for the very rare -500+s (vs -100+).
When I go to sea, don't fear for me, Fear For The Storm -- Birdie and the Swansong (tSCoSI)
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