When to double

Last night I played the 12 board EBU pairs with John Faben. He agreed to play my system, with the inclusion of the Culbertson New Asking Bid. This is a rather complicated method of slam bidding, which of course didn't come up at all. 

Instead our score was decided by a combined poor defence to 2NT, and a couple of misjudgements from me

On this one I really should have doubled, but didn't:

The auction strongly suggests a 4-4 Spade fit, so I know they are breaking 5-0. I also have lots of other defence and I'm not worried about them running to 5♣. So although it's John who has the trumps it's me who knows the whole picture, and should have doubled it.

I thought we'd get a good score just for beating it, but actually just 56%, as 4♠-2 was a common score.

Now here's one where I definitely shouldn't have doubled, but did:

It's a messy auction. I opened 1NT and John transferred to play in his long Diamond suit. Along the way North doubled Clubs then bid his Spade suit. When 3♠ came back round to me I know they've got to a silly contract, so doubled.

As it happens, my partner has two tricks for me (a Heart ruff and the ♠K), and we get 99% for 3♠x-2. But if I hadn't doubled, we still would have got 96% for 3♠-2, and I had no reason to expect two tricks in my partner's hand, so 3♠ might have made. Worse than that, my double could help them run to a better contract. If South bids 4♣ (and I think he would have if he was confident North's double shows Clubs) that's only one down;, and 4♥ is making. So my double had not much to gain, and a lot to lose.

Overall we finished on 61%.