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To me, "equal vulnerability" means both sides vulnerable or both not.

In sacrificing against a major suit game, "down two" means -500 when vulnerable versus -300 when not. This compares against opposing games (plus bonuses) of 620 and 420 respectively. In either case, you are saving 120 points.

But if you go down one (undoubled) against a part score, it's -50 not vulnerable, and -100 vulnerable. Counting a part score bonus of 50, either result represents a savings against a part score of 60+50=110. But down one vulnerable represents a loss against a part score of 40 (plus the 50 bonus for 90).

So the two "equal vulnerabilities" are "equal" against a game but not against all part scores. Is this a large enough factor to bid differently when both vulnerable than both not? This question is probably more applicable at matchpoints and perhaps IMPS than rubber.

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The equal vulnerability is only talked about at game level or higher. I haven't heard it being used for partscore battles, where it does not make too much sense, as you say.

Playing matchpoints, in partscore battles, the deciding factor is whether you are vulnerable or not.

-100 (down 1 double, non vul) might lead to a great score, but -200 (down 1 doubled, vul) is usually a poor score at matchpoints.

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    Even at game level, it can make quite a difference if you're playing IMPs. A close save against 4M is more appealing at all white, where being "wrong" means you only lose 80 points (420 vs 500); at all red, you lose 180 points (620 vs 800).
    – amalloy
    Aug 14, 2014 at 4:27
  • @amalloy: In your example, you are down three, which is a lot to pay for a saved game (except at favorable vulnerability). In my example, you are down two, a more "normal" level.
    – Tom Au
    Aug 14, 2014 at 13:16
  • @Aryabhata: People don't usually double for a one trick set (or do they)? Because if they do so at two of a major, they also double you into a potential game.
    – Tom Au
    Aug 14, 2014 at 13:19
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    @TomAu: The doubling is for when your side has the majority of strength and they have sacrificed at the 3 level, and at matchpoints, -140 might be as bad as -730 (they make doubled) or +100 (they go down 1), when most other tables are going +140. You double to convert the +100 into +200, beating most other tables in +140. Basically doubling has little to lose (-140 to -730 will be similar score), but a lot to gain (+100 to +200). Of course, this assumes +140 at other tables. Here is an interesting article you might like: bridgewinners.com/article/view/matchpoints-do-i-need-to-double
    – Aryabhata
    Aug 14, 2014 at 17:22
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    @TomAu The point is that if you're wrong about whether the save is profitable (ie, being down three when you hoped to be down two or less), that's more expensive at all red than at all white.
    – amalloy
    Aug 14, 2014 at 18:32
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In part-score bidding the most competitive is neither side vulnerable because the penalties for either side going down are lower, so playing rather than defending will be favourable unless both sides are going down or someone is doubling and the contract is going at least 2 off.

Thus actually in part-score bidding they are both opposites, the middle ground is one-side vulnerable, where you will consider not bidding either because you can get a better penalty defending (they are vulnerable) or your own penalty if you go down will be too big (you are vulnerable) and then it is really whether or not you think your side "owns" the hand.

In general:

Neither side vulnerable: Compete to the max Your side vulnerable: Bid on if you think your side owns the hand. They are vulnerable: Bid on more if you think they own the hand. Both sides vulnerable: Be more cautious

This is assuming matchpoints, that you can judge how far to compete and you can ascertain who owns the hand.

Of course if you can double a non-vulnerable part-score for 2 off you should prefer that to bidding any part-score of your own.

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