Scratchings are a frustration for any punter. Just when you think you have a race nailed, something pulls out and throws your best-laid plans out the window.

And at this time of year, with big fields and “inconsistent” weather (in Melbourne at least!), they can happen relatively often.

If you’re framing your own betting markets, or using those prepared by a professional punter like Trevor Lawson, Nathan Snow or Adam Mintz, it is important to understand the impact of scratchings and how they affect the market and the chances of each runner. It’ll allow you to quickly re-calculate your market in the event of a scratching and still find value bets.

To understand this, we firstly need to look at how a market works and how it’s calculated.
We’re going to use the example of Race 7 at Morphettville last Saturday, as rated by our South Australian analyst Adam Mintz.

Adam hasn’t rated no. 14, Alertness. This simply means he gives it basically no chance (0%) of winning.

The focus of any market is the prices, or the odds. Each runner’s odds are simply a representation of its chances of winning. It’s easy to convert odds to a % chance of winning: just divide 1 by the odds.

For example, number 2, Ample on Offa, is rated a \$7.10 chance by Adam. In percentage terms, that’s 14% (1 / 7.1 = 0.14, or 14%). Do this for all runners, and you’ll have the chances for each.

You’ll notice that the percentage chances of all runners don’t add up to 100%: here, it’s 88%. That’s because Adam is a professional punter and so he adjusts the percentages down from a 100% market to build in a margin in his favour – in this case, 12% (100% – 88%). In theory (and Adam has proven in practice as well) the difference between his outlay and his collect is his margin.

It’s the exact opposite of how bookies build in their own margin by rounding their markets up to say, 120%, which ensures them a theoretical margin of 20% on all bets.

On Saturday Adam’s rated favourite, Manhattan Melody, was scratched before the race, which meant every other runner’s chances of winning improved. To adjust the market to quantify this, you just have to redistribute Manhattan Melody’s chances (16%) across all other runners.

You do this by increasing each runner’s chances by 16% which, to cut to the chase, simply decreases their decimal odds by 16%. This rounds the market back up to the desired 88%.

Ample on Offa’s \$7.10 rated price becomes \$6.00 in the post-scratching market. He was able to secure odds of \$8.40. This wasn’t a sufficient overlay for betting on the original \$7.10, but it was on the re-stated \$6.00.

It’s that easy – except when they’re scratched at the gate!

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