Interest in determining which sires of race horses transmit raw speed, and which sires transmit stamina (defined as the ability to successfully compete at longer distances) to their progeny dates back to the early 20th century, when a French researcher, Lt. Col. J. J. Vuillier, published a study on the subject (called Dosage), which was subsequently modified by an Italian breeding expert, Dr. Franco Varola, in two books he authored, entitled Typology Of The Race Horse and The Functional Development Of The Thoroughbred.
[citation needed] However, these observations attracted little interest from the general public until 1981, when Daily Racing Form breeding columnist Leon Rasmussen published a new version of Dosage developed by an American scientist and horse owner, Steven A. Roman, Ph.D., in his analysis of the upcoming Kentucky Derby for that year.
[1] The new approach, which was more accessible to owners, breeders and handicappers and was supported by solid statistical data, rapidly caught on, and the term "Dosage Index" has been a fixture in the lexicon of horse racing ever since.
High Dosage Index (and Center of Distribution) figures are associated with a tendency to perform best over shorter distances, while low numbers signify an inherent preference for longer races.
In recent years, however, several horses with no Solid or Professional chefs-de-race in the first four generations of their pedigrees—and indeed, a few with Dosage Indexes of above 4.00—have managed to win the Kentucky Derby and Belmont Stakes, highlighting the issue of increasing speed and decreasing stamina in contemporary American thoroughbred pedigrees.
Yet the statistical foundation of Dosage remains compelling and the theory accurately differentiates Thoroughbred pedigree type for large populations of horses competitively performing over a range of distances, track surfaces and ages.