What causes draw bias?

What causes draw bias? In many racing jurisdictions, including Britain and Ireland, the vast majority of Flat races are started from electromechanically-operated starting stalls. The purpose of starting stalls is to allow an even break, where participants start on level terms, in as straight a line as possible. Starting stalls are numbered, from left to right if viewed from behind, and stall numbers are drawn, at random, by Weatherbys, which provides administrative services to the British Horseracing Authority (BHA) on the day on which declarations are made.

An even break is one thing, but various other factors may introduce draw bias, such that horses drawn on one part of a racecourse hold an advantage over those drawn elsewhere. These factors include the design and characteristics of the racecourse, including the racing surface, its level of usage and, of course, the weather.

Some parts of a racecourse may drain quicker than others after rainfall, creating a disparity in going across the width of the track. Similarly, the use of movable running rails has become increasing commonplace in recent years. This can have the effect of creating a ‘golden highway’ of fresh ground next to the rail, such that horses drawn on that side hold an advantage. The location of the start and the position of the starting stalls may also create draw bias, one way or another. If the start is located close to a bend, the horses drawn on the outside need to travel further than those on the inside and are naturally disadvantged.