Sucker said:
Ok; accepting your figure of a 2.1% gain on the initial bet; here is what is REALLY happening:
If you put up another max-bet to complete the double-down; yes - the return on the INITIAL bet is 2.1%. HOWEVER, you are ALSO earning 2.5% on that DOUBLE DOWN max bet. For a $200 bet you are earning $4.00 by not doubling, but you will be earning a total of $9.20 by doubling. This works out to a gain of 2.6%, not .1%.
Sucker,
I am glad to hear that we are on good terms in the academic arena.
Now permit me to explain why your reasoning is incorrect here, as relates to to this example.
You are confusing TBA (Total Bet Advantage) with IBA (Initial Bet Advantage), and therein lies the problem. You are not considering the fact that the advantage on your hand changes when you decide to split or double-down. In fact, you are making it less likely that you'll win the hand, at the exchange of being able to put more money on the table. In the example I gave, the advantage relates to your initial bet.
So, back to the example. You'd have a 2% advantage
with respect to your initial wager on your hand if you decided to take a hit (and play your hand from there. However, if you were willing to put up another max-bet by doubling-down, then you'd have a 2.1% advantage,
with respect to your initial wager.
However, by doubling-down, you do not have an advantage of 2.1%,
with respect to your total wager (IOW, your initial bet plus double-down bet). Indeed, by doubling down, you are decreasing your chance of actually winning the hand, and thus,
you are decreasing the advantage with respect to your total wager. However, so long as you are not decreasing your advantage by more than a factor of 2, then the EV is still positive because you are doubling your wager. Therefore, the EV-maximizing index is determined by the point where you are giving up less than half of your edge to be able to double your wager. With this enlightenment, let's revisit the example.
By taking a hit and playing our hand from there we have an advantage of 2% with respect to our initial wager, but if we are willing to double the money on the table then we'd have an advantage of 2.1%, with respect to our initial wager. This then means that our advantage on the hand has only become 1.05%, but by doubling our wager, we get a 2.1% return with respect to our initial wager.
So, if we assume our max-bet is $200, we have an expectation of acquiring $4.00 by not doubling (since we have a 2.0% advantage). However, if we put out another $200 to double-down our total wager becomes $400, but doubling decreases our chance of winning the hand to 1.05%. Therefore, by doubling we expect to earn $4.20. Hence, we have jeopardized another $200, just for an additional expectation of $0.20.
As you can see, your advantage decreases by doubling, but so long as you are able to double your bet and your advantage decreases by less than a factor of 2, then the EV will be positive. However, looking at things from only an EV perspective leads to some non-SCORE maximizing outcomes that greatly increase RoR for a disproportionate return.
Furthermore, TBA (Total Bet Advantage) is not to be confused with IBA (Initial Bet Advantage).
Best,
SP