Horse Racing Non-Runners

In Horse Racing terms, any horse declared for a race which does not subsequently come under starter's orders is deemed to be a non-runner. Prior to all betting on a race, each horse is given a 'reduction factor', based on Betfair's estimate of each horse's chance of winning with reference to the racing press - this number can be retrieved using the API via the MarketInfo field in the response to the MarketRefresh call. In general the higher the number the more fancied the horse. Reductions will be made to both win and place markets but applied differently, and horses will have a different reduction factor for each.

On win markets the reduction factor is applied to the total odds. Thus, if a reduction factor of 12% is applied to odds of 5, the resulting reduced odds would be 5 - (12% x 5) = 4.4. In this case, it is mathematically provable that the reduction factors must sum to 100%, if they are to be fair.

However, for place markets, the reduction factors are applied to the odds minus the stakes (i.e. the odds-1) so that if a 40% RF is applied to a bet at odds of 3, it gets reduced to 3 - ((3-1)*40%) = 2.2. In this case, the sum of the reduction factors do not sum to 100%, or any specific value.

The calculation to get these reduction factors is based on predicted odds (from a reputable source such as the Racing Post) and uses an iterative process to calculate the 'fair' reduction factor based on the effect of removing each of the runners in turn. In other words, it removes a runner and calculates what the reduction factor would have to be in order to return a 'fair' market back to 100%, after the removal. For this purpose a 'fair' market is assumed to be one where the market reflects the reduction factors precisely.