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Unders / Overs betting on AFL Football

(E-mail to AFL Insiders Forum from an AFL client)

-----Original Message-----
Sent: Tuesday, 10 June 2003 12:35 PM
To: 'Ozmium AFL Insiders List'
Subject: Overs / Unders

Just to stir the pot a little - I would caution people against punting on the Unders and Overs.

Background info - some bookies have a bet type where you can choose a side to win by Under/Over 39.5 and others have Under/Over 24.5.

I spent a couple of years trying to search for better payouts and trying to have an opinion on the margin but I have concluded this type of bet is just too difficult compared with Win and Line betting.

My point is purely mathematical. An individual bookie bets to around 104-5% on Win betting and 105.3% on the Line, whereas it is typically 108-9% for Unders/Overs. Less bookies bet on Unders/Overs and are split between the 24.5 and 39.5 margin so there is less potential for better odds by shopping around. I would estimate you would average 103% on Win betting, 104% on Line betting and 107% on Unders/Overs.

This puts a significant hurdle on the extra information you would need to have to make betting on Unders/Overs better value than betting on Win or Line, especially given that the margins of each cross over to some extent and your opinion on the outcome of Win or Line versus Unders/Overs is related (ie if you think one is good value then the other is very likely to be, too).

This point can be shown quite clearly when you consider that if you bet on an Under/Over you are deciding to avoid the option of taking a lower dividend and covering the other portion of the margin and well as half cover a drawn game if you had bet on the Win.

You can compute the implicit probability/dividend on this foregone portion of the game's margin. eg Western Bulldogs versus Hawthorn in Round 20, 2001 at odds of $2.70 for a win, $3.35 Under 39.5 and $10.50 over 39.5. By choosing Under 39.5, the break-even probability is 1/3.35 = 0.2985, and choosing the Win the break-even probability is 1/2.7 = 0.3704. The difference is 0.0719, which splits up into 0.005 for a draw (half the 100/1 probability of a draw) and 0.0669 for Over 39.5, which converts to a dividend of $14.95.

So that means that even though you think the market-determined dividend of $3.35 represents good value, the fact that you did not choose the odds of $2.70 for a win instead means that you think the true probability for a win over 39.5 converts to a dividend in excess of $14.95, well over the market-determined dividend of $10.50.

This would only make sense if there was a strong reason that you expected the Dogs to not win by a large margin, which would only be true in a few special circumstances. (Of course the Dogs won by Over 39.5 in this example.)

The same rationale can be applied to every game, where you can determine the implied dividend for the portion of the margin that you choose to forego. If you are willing to forego this margin for the dividend then fine, but at least do it knowingly.

Happy punting. D2.


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