Cribbage Pro player “sm15” sent me a message asking about the pegging advantage of playing a flush. That’s a great question, and got me thinking about flushes in general.
How likely is your opponent to have a flush?About 3.0% of dealer’s hands are flushes. Pone’s hand holds a flush about 3.3% of the time.
Pone’s initial lead gives a clue as to whether or not they have a flush. A lead 5 is only 1.4% likely to be part of a flush. A lead 2 or 3 is 4.0% likely to be part of a flush. Basically if a lead card is more likely to be part of a trap (see the “Should you pair your opponent’s lead?”), it’s less likely to be part of a flush.
How likely are you to have a flush in your crib?Remember that a flush in the crib only counts if the cut card is the same suit as the cards in the crib, so it’s significantly less common than a flush in the hand. Around 0.17% of cribs hold a flush (1 in 588). Even when dealer tosses two suited cards in her crib she still only ends up with a flush 0.81% of the time (1 in 123). When pone tosses suited cards in the crib she ends up giving dealer a flush 0.77% of time time (1 in 130).
Should you toss suited cards in the crib?If it’s not going to make a difference to your hand, you might as well toss suited cards as dealer, and off-suit cards as pone. Usually all things are not equal, though, and pone might be faced with the choice of splitting up her hand to avoid giving dealer a flush. Changing your hand to avoid throwing suited cards is a pretty subtle play. When pone tosses suited cards into dealer’s crib she has a 0.77% chance of giving dealer 5 points, which works out to an average of 0.04 points.
Do situations exist where that 0.04 would make a difference? Sure. When pone discards 8-6 she gives dealer, on average, 0.01 points more than if she’d discarded 8-9. But if pone’s 8-9 is suited it might make sense for her to toss an off-suit 8-6 instead. That would offset the extra points from the 8-6, penalizing dealer an average of 0.03 points. Those three-one-hundredths-of-a-point could win you the game, but you've probably got more important cribbage-related things to worry about.
The Jack is a special case. Imagine you’re playing as pone and your hand holds: J♣, J♦, 6♦, 7♠, 8♠, 4♦. The right toss is probably the 4 and a Jack. But which Jack? If you toss dealer the J♦/4♦ you’ll give her a 10/46 (10 diamonds left in the deck after the Jack, 6, and 4, and 46 cards left in the deck) chance of scoring 1 for his knobs, plus a 0.77% chance of giving her 5 points for a flush. That works out to about 0.256 points.
If you toss the J♣/4♦ you’re giving her a 12/46 (12 clubs left in the deck / 46 cards left in the deck) chance of scoring 1 for his knobs, which works out to about 0.261 points. Throwing the off-suit Jack gives dealer an extra 0.005 points that you’d just as soon she not have! So if you’re going to toss a Jack into your opponent’s crib, consider tossing the suit that shows up most often in your hand, even if it might give dealer a flush.
How big is the pegging advantage?To answer sm15's original question, the results depend on the hand being played, and who’s playing it (dealer or pone). For instance, we've seen that when dealers play a suited 6-7-8-9 they peg about 1% more points, while their opponents peg about 9% fewer points. A dealer holding suited A-4-Q-K will peg about 1% fewer points, while her opponent will peg about 7% fewer. And when pone holds A-4-Q-K she’ll peg the same number of points whether or not its suited, but dealer will peg 5% fewer points if pone’s A-4-Q-K is suited. (All of these statistics are based on several hundred examples of pone playing suited versions of those hands)
Taking into account every possible hand, dealer will peg about 1% fewer points if her cards are suited than if they’re not; and dealer’s opponent will peg about 4% fewer points if the cards are suited. If pone’s cards are suited, pone will peg the same as if they weren't suited, but her opponent will peg about 1.5% fewer points. So the pegging advantage to a flush is more about limiting your opponent’s pegging than it is about increasing your own.