An adage of modern bridge is to NOT let the opponents play on a fit in the two level, and to compete fiercely for the part score.
This is certainly true. However, there is a time where competing is more likely to lead to a bad result, and we should use caution.
When we have a decent amount of points in the opponents suit, declaring will often be a bad idea. It is in these times that we should be less inclined to compete.
A big scan was done of hands where competing was a bad idea.
It was almost always a good idea when the competing side had nothing or very little in their suit.
It dropped sharply in effectiveness as the competing side had more and more in their suit.
More points in their suit means declaring is harder (our trump suit and side suits have more losers, the points aren't where they need to be!)
It also means defending can be more profitable, our points are in their suit which is typically extra tricks in defence.
Below is a good sample of the hands where competing worked out poorly.
(The scan couldn't find any hands that worked out badly when the competing side had little or nothing in the opponents suit! i.e. when you don't have wastage in the opponents suit, it's a great idea to compete.)
These are hands where competing led to a worse score
Circled: the points held in the opponents' suit.
Result: 3♣X three down, -500. Leaving them in 2♠ would have been -140. Cost: 8 IMPs.
Result: 3♦X two down, -500. Leaving them in 2♠ would have been -140. Cost: 8 IMPs.
Result: 3♥X one down, -200. Leaving them in 2♠ would have been +100. Cost: 7 IMPs.
Result: 3♦ three down, -150. Leaving them in 2♠ would have been +100. Cost: 6 IMPs.
Result: 3♣ four down, -200. Leaving them in 2♥ would have been +50. Cost: 6 IMPs.
3♣* Artificial, showing diamonds.
Result: 3♦ two down, -100. Leaving them in 2♠ would have been +100. Cost: 5 IMPs.
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