Total matched bets for 'back' versus 'lay'

[lonestar79]

Hi all,

I am using the listMarketBook function of API to download the offered prices in the exchange for a game. This function shows the total matched amount for each selectionID; however it does not break down this matched amount as the total matched on the 'back' side of the market versus the 'lay' side. Is there a way to get this information please?

[Merlin]

Hi lonestar,

The total is the total of 'back' stakes matched - if you need to find the total 'lay' value, you need to call listMarketBook with the priceData parameter of priceProjection set to EX_TRADED and do the multiplication yourself.

cheers

Merlin

[lonestar79]

Thank you Merlin, that makes sense.

[lonestar79]

Can I ask a follow up question on this please?

For a particular market, I call listMarketBook many times a minute with priceData parameter of priceProjection set to EX_BEST_OFFERs. Hence I observe the change in the amount offered at the best back price over very short periods of time. I then observe the change in matched amount for the same back price (using EX_TRADED). The matched amount is exactly 2 times the change in the offered amount. Why would this be? I suspected that there is a bug in my code which results in double counting but so far I have not found any. Is there a reason why matched amount would be 2 times the change in offered amount?

[betdynamics]

You are dealing with an Exchange, so matching a bet involves two transactions - one a back; the other a lay.

If I place a £2.00 back bet at a given set of odds and the bet is matched, then it means that I have been matched against someone else who layed at those given odds.

So, there is one matching back bet for £2.00 and one matching lay bet for £2.00 - hence why there is an increase of £4.00 in the total matched value.

[lonestar79.]

Betdynamics,

Thank you for the answer. Just for my clarity, can I check one detail: To my mind, the amount matched depends on the odds where the match happened. If you backed £2 at an odds of 3, then the other side to this match is risking £4 against you. So the amount matched on back side is £2 and lay side is £4, no?

It seems that the tradedAmount field assumes that both sides are a £2 pound match though. (when listmarketbook function is used with price projection=EX_TRADED). This is the source of my confusion. I can live with the output being as it is but I just want to double check that I am not missing anything in my understanding above.

Thank you for any help.

[betdynamics]

I should have said £2.00 liability on the lay (rather than a £2.00 bet).

You are matching like for like - so my £2.00 will be matched against someone else's £2.00 liability. It doesn't have to be the entirety of their bet, just £2.00 of their liability.

[lonestar79.]

Understood. Thank you for answering.

[Nick JD]

Along these lines, I found this simple, short video great for explaining BF's cross-matching.

https://www.youtube.com/watch?v=Bys1lu-18wo

[lonestar79.]

Hi Betdynamics,

I just noticed that sometimes the change in the matched amount is not 2 times the change in amount offered at best back price, but rather equal to 1 times the change in amount offered. So I am back to being confused now... Any idea why this might be happening? Most of the time, the "2 times" rule seems to hold but every once in a while it doesn't.

[betdynamics]

Is the change always even?

If so, then it is always two times something, so the two times rule still stands.

It could be 2 x a cross-matched odds value, rather than the currently displayed odds value.

[lonestar79.]

Not always even unfortunately... How can I get a definitive answer on this? Is there some email contact of Betfair developers that I could try?

[Lopiner]

I have also noticed this, it would be very interesting to see this answered.
You can try bdp@betfair.com

[jptrader]

I have also noted this, and I am quite sure it happens after cross matching. E.g. if the lay side is matched, but not against a back bet on the same selection, but by a cross matching bet, the matched amount reported for that selection is only 1 times the matched amount. (Which makes sense as there was no back bet on that selection)

[Lopiner]

I also think that's the most probable scenario jptrader, it makes sense.