When Steve Roman, a Texas mathematician, started using computers to analyze thoroughbred pedigrees, he couldn't have imagined the byproduct his work would yield. In the course of his research, Roman uncovered a spectacularly successful way to pick winners (and losers) of the Kentucky Derby.

Roman's refinement of an old breeding theory called the "Dosage System" assigns every horse a number that reflects his distance-running ability. The lower this Dosage Index, the better the animal is suited for longer distances.

Roman found that a horse needed a Dosage Index lower than 4.0 to win at 1 1/4 miles. In the last 40 runnings of the Derby, every winner has met this standard.

This year, Roman's numbers will be put to a severe test, for they contradict the opinion of most expert handicappers. His system says that Chief's Crown, the standout favorite for Saturday's race, doen't have the genes to win at 1 1/4 miles. Nor do a couple of the strongest stretch-runners in the field.

Roman based his work on the method of an Italian breeder, Franco Varola, who analyzed pedigrees by counting the number of dominant stallions -- which he termed "chefs de race" -- in the last five generations. Roman refined this approach by putting all the "chefs" in five different categories that reflected their tendency to pass on speed or stamina.

Roman uses a point system to count the influences of speed and stamina. The ratio of speed to stamina is called the Dosage Index.

Roman's method has not won a legion of followers within the breeding industry. He has plenty of detractors, such as Bill Oppenheim, editor of the newletter Racing Update, who flatly declares, "Dosage is a waste of time." But the Dosage System began to win attention and gain respectability because of its record in the Derby.

For whatever reason, the theory that a horse needs a Dosage Index of less than 4 to win at 1 1/4 miles has held up year after year. In 1982, only three horses qualified; Gato del Sol won at 21 to 1 odds and Laser Light ran second at 18 to 1. Last year, seven of the entrants, including the favorite Althea, failed to qualify. All of them finished 10th or worse.

This year's favorite, Chief's Crown, has looked in his 1 1/8-mile races as if he could keep going forever, but the system says no. His Dosage Index is 5.0.

Of the other principal contenders at Churchill Downs, two formidable-looking stretch runners are eliminated by the Dosage System: Skywalker, the Santa Anita Derby winner, who has a 7.80 index, and Tank's Prospect, the impressive Arkansas Derby winner, with a 9.40. Eternal Prince, who captured the Wood Memorial, doesn't make it, either, with his 7.0.

The Derby contenders with the proper pedigrees are Proud Truth (0.74), Spend A Buck (1.4), Stephan's Odyssey (2.53) and Rhoman Rule (3.57). Roman prefers Stephan's Odyssey and Spend A Buck.

Critics of the Dosage System would love to see it flop in the Derby, since its record in the race is its greatest claim to legitimacy. The anti-dosage people have been waiting a long time, in vain, for such a failure, but Oppenheim said, "I'm going to double my exacta on Tank's Prospect and Chief's Crown just because the numbers say they can't go a mile and a quarter."