In a horse race, the favourite is the horse thought most likely to win, yet does so on just one-in-three occasions, on average. One reason for a high proportion of losing favourites is that roughly 60% of horse races run in Britain are handicaps or, in other words, races in which every horse is allotted a weight which, at least in theory, gives it an equal chance of winning as any other horse. Another is that, unlike, say, the roll of a die, in which the chances of rolling a particular number is, unequivocally, one-in-six, or 5/1, the odds offered on a horse race inevitably involve an element of human opinion.
A bookmaker may initially calculate the probability of a favourite winning based on its form and other information in the public domain, and post odds accordingly, but those odds will be affected by the opinion of other bookmakers and the betting public. The betting public voices its opinion by accepting, or rejecting, the odds offered and, in effect, ‘builds’ the market for the race in question. Of course, public opinion, like that of the bookmakers, is not always correct, but can sometimes lead to a horse being backed – in the absence of an informed alternative – simply because it is ‘favourite’ for the race in question.
Of course, horse racing is subject to a myriad variables, including, but not limited to, course, distance and going, plus an infinite number of imponderables – not least of the horse, itself, as a sentient being, capable of emotions such as anxiety and fear – so it should come as no great surprise that the horse that ‘should’ win a race fails to do so on a regular basis.