DSIP when we know we have a fit

The question has recently come up: which is better when we know we have at least an eight-card fit?  Penalty doubles (as recommended by Robson-Segal and as generally played by most players) or two-way (DSIP) doubles?

You won't be surprised to hear that I believe two-way doubles are best, although generally I agree with everything that Robson-Segal say in their excellent book Partnership Bidding: the Contested Auction.  But rather than simply go on gut feel, I decided to put it to the (theoretical) test.  I developed a spreadsheet which examines the question in an objective manner (I hope).

The result is that DSIP is better by an average of about half an imp or so.  I'll try to show the pertinent results here:

Unfortunately, Google Docs can't handle the spreadsheet but let me know if you'd like a copy.  Here's the basic idea:

• Scenario: Equal vulnerability. S (dealer) has a reasonably balanced hand with five spades and opens 1♠, N responds 2♠, E bids 3♣
• South's options are: pass, or double (penalty in R-S case, two-way in DSIP case);  minimum point counts are S: 12, N: 6.
• North may or may not have four spades.  His options are pass or double (same as South).

There are several observations to make.  Firstly, we all agree that when a 9-card fit is known, doubles should be for penalty (9-card fits are frequently diagnosed with an initial jump, otherwise if a two-way double occurs, then the partner with extra length will go back to the known fit suit, and thereafter doubles will be for penalty).

Secondly, if either North or South had a distributional hand, there would be no problem (especially if the partnership is playing good-bad 2NT).

Thirdly, these doubles are not game tries per se (although the "maximal" double would be a game try if RHO had bid 3♥).  The primary purpose of these doubles is to compete, despite not having a distributional hand.  Nevertheless, since they show extras, they will result in a game being bid sometimes.  In the spreadsheet, this occurs once, where N/S are both short in clubs and both have extras.

The essential premise of the DSIP method is that you can compete or penalize when either player has both extras and shortness in the enemy suit.  I have defined extras as having a Queen over the minimum.  Shortness is typically a small doubleton.  When playing penalty doubles, either player has to have both extras and a "stack" in the enemy suit.  The player sitting over the length might be able to make a penalty double with a stack but no extras but the player under the length may not feel comfortable doubling even with extras.  In three cases (two where North makes a penalty double and one where he passes his partner's two-way double) I have adjusted for this by saying that half the time he will do something else that is not so lucrative.

Finally, an example (a minor modification of a hand from last night's IMP game).  Vulnerable vs not, you pick up in fourth seat:  ♠AQ42 ♥AQ543 ♦K8 ♣62.  After three passes, you open 1♥ and partner bids 2♥.  RHO now comes in with 3 ♣.  You have shortness in clubs, a K extra and so you double.  You weren't thinking of bidding 3♠, were you?  Partner has made a slightly conservative raise with ♠JT9 ♥JT76 ♦AQ97 ♣Q9.  Still, he has a full 8 count outside clubs (and it looks like the opponents have a 9-card fit) so he bids 4♥.  This is good for 3.86 imps.

What it comes down to is this: which is the more likely holding in the enemy suit?  Shortness or a stack?