Easier to calculate than to overcome
Whilst it can be debated whether the Random Factor may or may not have an unduly large influence on the outcome of races it is an absolute certainty that weight is a key factor in affecting race results.
Simply put, if your horse is due to carry more weight than other horses in the race you have just handed an advantage to those lower-weighted horses in the race.
In Stakes racing if you are unsure how to calculate the racing weight for any horse in any particular race you can determine what weight your horse is due to carry in a race by using the 'Calc weight' option on the Race Nomination. It is generally advisable that your horse carries the minimum possible weight in Stakes races although it can sometimes be better for a horse to carry a penalty in a lower division than to move up a division and face horses with greater ability.
In Rated racing it is not quite so simple. Racing weights for this scheme work on a comparitive system as opposed to the absolute system in Stakes racing. The only time you can be sure of the weight a horse will carry in a Rated race is when that horse has the maximum rating allowable for that race, in which case you can be certain it will carry 140lbs. Racing weights are calculated by allowances being granted for every ratings point below the top-rated horse in the race, that horse or horses being automatically alloted 140lbs. The top-rated horse in any race may or may not have a rating equal to that of the maximum allowed so you cannot simply assume that because your horse is rated 4 points below the maximum for that race it will receive an allowance for 4 ratings points. Until the final field is declared you will generally not know exactly what weight your horse will be given.