Betfair SP - How Exactly Is It Calculated?

[Aitch]

Howdy folks


I gather that Betfair use a pretty sophisticated formula to determine their SP to include all the unmatched backs and lays?

How does it work if someone places a very large back bet, right before the start of a horse race?


For example, in a typical horse race where about 500K is matched, and the Favourite is priced at 3.0 BSP...

What would happen to the BSP if someone put 5K on the Favourite just before off?

Would the BSP drop only slightly from 3.0 to 2.9 - or would it be slashed down to odds on or even less?

[Dallas]

https://promo.betfair.com/betfairsp/FAQs_SPrules.html

[Euler]

The problem with SP is that it is often resolved in reality just after post. So if a horse gets a good start or rears in the stall, the backers and layers will slam the market and distort SP.