First let's look at paralays. A standard, two team, winning parlay at a reputable book pays 2.6 x each unit wagered. Also there are 4 possile outcomes (2^2). In the case of team A and team B hooked to the Over and Under, the 4 possibles are: team A/over, team A/under, team B/over, team B/under. Suppose from the book's standpoint that 10 units are wagered on each of the 4 possible outcomes, which equals a total wager pool of 40 units. Ignoring pushes, the book pays the winning player 26 units for the win plus returns the original 10 unit wager (10+26=36) resultng in a profit of 4 units for the book. The vig in this case is 4(profit)/40(total wagered) = 10%.
On the other hand most local books don't pay 2.6 but rather something like 6 pays 14. Following the above logic, wagering 6 units on each possible leg (6 to keep math in integers), the book books 24 units in action and returns 14+6=20 to the winning player. As before the profit to the book is 4 units but the vig is 4/24 (as opposed to 4/40) = 16.66%
As discussed in other threads for the player to win at 10% juice, he must cover ~53% of his plays. That is just barely breaking even. The actual calculaton for this is betting 110 to win 100 =
110(wager amt)/210(collected amt) = 52.38%.
Using the numbers for the reputable book above, the break even point =
10(wagered)/36(collected) = 27.77%.
Now looking at actual data from 2008-2009 NBA season. Tossing out games with
pushes, here is the breakdown of winning parlays in 1196 games:
300/1196=25.08% fav/ov
291/1196=24.33% fav/un
300/1196=25.08% dog/ov
305/1196=25.50% dog/un
and so it goes...