The Double for Less Shift

Discussion in 'Blackjack Tournament Strategy' started by BlueLight, May 31, 2018.

  1. BlueLight

    BlueLight Active Member

    Almost anyone who visits Blackjacktournaments.com knows about Smith's quandary
    of Doubling for Less when trailing as in Example(1) below shows.

    Infinite Deck...................Last Hand..................Min Bet = 5.....................Max Bet = 500

    Player..........BkRoll...................Bet...................Cards

    BR2...............1000...............500 + 250..............2+3
    BR1................1195.....................500.................10+10

    Dealer....................................................................7

    BR2 bets the max 500 and BR1 correlates by a matching bet of 500.
    BR2 now doubles for 250.

    The BR2 double is right in the middle of a double for 0 and a double for 500.
    The BR2 double cannot be correlated by BR1 by splitting since he cannot split for less.

    Actually any BR2 double of 200 up to 300 will put BR1 in the quandary.
    If BR1 stands he gives up the high to BR2.
    If BR1 splits he gives up the low to BR2. What should BR1 do?

    Below are the probabilities of BR1 winning the session by the simple but reasonably
    good strategy of splitting and playing not to bust against various dealer up cards.
    Also given are the dealer bust probability with the various dealer up cards.

    Dealer Up Card.............................................7..........................8......................10

    Dealer Bust.............................................. .2623.................. .2447............... .2298

    Prob of BR1 winning by
    splitting and playing not to bust............ .8004................ .. .7818............... .7492

    The best way for BR1 in Example(1) to play against the quandary with a dealer 7
    up card is figured as follows:

    The probability of BR1 winning by standing is BR1 needs the Dealer to make a hand.
    Prob(Make a Hand) = 1 - Prob(Bust) = 1 - .2623 = .7377.
    Since .7377 for standing is less than .8004 for splitting and playing not to bust;
    then splitting and playing not to bust is the better way to go.

    Instead of a dealer 7 up card in Example(1), what is BR1's best line of play against
    an 8 or 10 dealer up card and each of their probabilities?


    ..............................................................BlueLight
    as follows:
     
    gronbog likes this.
  2. BlueLight

    BlueLight Active Member

    After 2 weeks nobody has tried to solve the teasers when the dealer has an 8 or 10 up.

    Player................BkRoll..........................Bet..............................Cards

    BR2....................1000......................500 + 250..........................2+3...............Doubles
    BR1.....................1195............................500.............................10+10..............Action?

    Dealer............................................................................................8

    To solve the problem for BR1 standing when the dealer has an 8 upcard you need to
    know the probability or percent chance the dealer will make a hand.

    When the dealer has an 8 upcard he has the following total outcomes (that can also
    be found in other sources) :

    Dealer makes...................................probability...............................% chance

    ........21................................................ .0694 ...................................... 6.94
    .......20................................................ .0694 ...................................... 6.94
    .......19................................................. .1286 ..................................... 12.86
    .......18................................................ .3593 ..................................... 35.93
    .......17................................................ .1286 ...................................... 12.86
    .....Bust.............................................. .2447 ...................................... 24.47

    ..............................................Sum = 1.0000.........................Sum = 100.00 %

    Note that to account for all the possible outcomes the probabilities must add up
    to 1.0000 or the percent chances must add up to 100%.
    (I have seen some tables[but not at blackjacktournaments.com] where the sums
    did not add up to 1 or 100%.)

    We could add the chances of the dealer making a hand, by adding the chance of 21,
    the chance of 20, the chance of 19, the chance of 18 and the chance of 17.
    However by simply subtracting the chance of the of the dealer busting from 100%
    (or subtracting the probability of busting from 1) we get the total of the made hands
    probabilities.

    Dealer makes a hand = 1.0000 - dealer bust = 1.0000 - .2447 = .7553

    for the situation in example(1) when the dealer upcard is an 8.

    (From the table given above for splitting)
    BR1 Splitting and playing not to bust yields .7818
    .......................................BR1 standing yields .7553

    Therefore BR1 splitting and playing not to bust is the better way to go.



    When the dealer upcard is a 10:

    Player................BkRoll..........................Bet..............................Cards

    BR2....................1000......................500 + 250..........................2+3...............Doubles
    BR1.....................1195............................500.............................10+10..............Action?

    Dealer...........................................................................................10


    Here the dealer with a 10 up has a better chance to make a hand.
    BR1 standing with 10+10 is better than splitting and playing not to bust.

    Splitting and playing not to bust above was .7492

    For BR1 standing, BR1 needs the dealer to make a hand.
    The probability of the dealer making a hand with a 10 upcard is :
    1 - Prob(Bust) = 1 - .2298 = .7702

    BR1 splitting = .7492
    BR1 stands... = .7702

    Therefore when the dealer has a 10 upcard then BR1 standing is the better way to go.

    ***However the teaser asked for the best line of play. Once it has been determined
    that splitting is not so good when the dealer has a 1o upcard, BR1 has a simple line
    of play which is easy to calculate that squeezes out and additional 1.77% chance (or
    0.177 probability) over standing.

    What is that BR1 line of play.


    .......................................................................BlueLight
     
  3. Monkeysystem

    Monkeysystem Top Member Staff Member

    The rule of thumb, then, is split against dealer 9 and below, stand against dealer T and above.

    You're playing two hands each with a starting hand total of 10. So naturally it seems you should be the favorite in each hand against dealer upcards of 9 and below, but not so much against a dealer T or A.

    It seems like common sense, which makes it exceptional in the world of blackjack tournaments! :D
     

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