For our horseracing markets we guard against possible non-runners with a system of Reduction Factors (similar to Rule 4 with traditional bookmakers). Each horse is allocated a Reduction Factor, based on its likely chance of winning the race. Should a horse be declared a non-runner, the Reduction Factor will be applied to the prices of all matched bets, for both backers and layers.
This is to ensure that layers are not unfairly treated and exposed to large liabilities when the chances of other horses winning have improved. The Reduction Factors are designed to be fair to both backers and layers.
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