The popular conception of the role of the handicapper appears to be that he adjusts his weights (ratings) according to the winning and losing margins, thus reducing the whole operation to a mathematical exercise.
Although there is certainly a mathematical and scientific basis regarding the principles of assessing and handicapping racehorses, it is not that simple in practice. The handicapper might have done his job to the best of his ability when assigning the weights - in theory all the entries should dead-heat (an impossibility in fact), or at least should have an equal winning chance. However, there are many factors over which the handicapper has no control and which can influence the outcome of a race. He has to take all these into account when evaluating a race performance. Remember, the handicapper does not just assess handicaps; he will have to assess every race and come up with a rating he can substantiate.
We have already mentioned ground conditions from the mathematical point of view but, apart from the weight factor, the main reason most racehorses do not always perform to their best is that they are unable to act on the prevailing ground. All horses should act on good going but some may be more at home on firm or soft and, as a result, perform markedly better on whichever surface suits them. Obviously, the form student should readily identify those horses and should be prepared to make allowances when, for whatever reason, they do not have their ideal conditions underfoot.
The other significant factor regarding a horse's performance relates to distance. The pedigree of a horse, and to some extent its temperament, should give the owner and trainer an idea of the likely preferred distance and most will sensibly attempt to race the horse over a suitable trip with reference to those factors and how it has performed on the racecourse to date. The handicapper will also note those horses that are being asked to race over an unsuitable distance.
The draw, particularly over sprint distances, can also have a significant effect. However, any perceived advantage from a 'good' draw can be negated if the horse starts slowly and sometimes, even from a 'bad' draw, a horse can be held up for a late challenge if that is what suits him and still take a hand in the finish. Again, the handicapper will note the draw and any likely advantage or disadvantage.
Other important factors that can have a bearing on the outcome of a race are jockeyship and tactics. Sometimes a race is not 'truly run' and a slow early pace with a sprint finish can sometimes produce a 'false' result.
Fitness, particularly in the early part of the season, can also be significant.