Sports fans who love statistics usually become baseball junkies because of the wealth of numbers the game provides. But baseball stats are child's play compared to those in thoroughbred racing and breeding.

This is more than a game. Hundreds of millions of dollars are spent every year on yearlings, stallion shares and stud fees, and the prices depend largely on the quality of the sire. But how do you quantify such results?

Traditionally, racing statisticians have ranked horses according to the earnings of their offspring. Earnings figures, however, can be confused by inflation.

They can be knocked further out of whack by international racing -- purses in England are so much lower than in North America that a British champion may earn less than a low-grade New York stakes horse. Earnings figures became utterly preposterous in 1985, the year of million-dollar purses, the year of lavish bonuses, the year of Spend A Buck.

When Spend A Buck earned $2.6 million for winning the Jersey Derby, his sire, Buckaroo, was propelled to the top of all the stallion rankings. According to figures compiled by such authorities as The Blood-Horse, the Thoroughbred Record and Bloodstock Research Information Services, the previously unheralded Buckaroo is a better sire than Northern Dancer (who is, in fact, the most influential, sought-after sire in the world). Said Dave Dink of the Thoroughbred Record: "There have been some rumblings that we need a better, more definitive system because of -- I hate to use the word -- the Buckaroo fiasco."

Whoever comes up with one definitive measurement for stallions -- something comparable to Tom Boswell's ultimate baseball stat, Total Average -- can hang up his shingle as a bloodstock consultant and become rich quickly. In the meantime, the industry is using a wide variety of stats -- the Average Earnings Index, the Sire Production Index, the Comparable Index, the Annual Production Earnings Index.

The most durable measurement of stallions was conceived by Joe Estes, publisher of The Blood-Horse, in the inflationary year of 1948. Estes wanted to eliminate the effects of inflation on horses' earnings figures, and he did this by calculating the average earnings of every North American racehorse in a given calendar year. Then he would compare this number with the average earnings for all the progeny of a given sire.

In 1985, the average runner earned $7,778, and a stallion whose average runner made exactly that amount would have an Average Earnings Index of 1.0. If the stallion's progeny earned $15,556 apiece, his AEI would be 2.0.

The Blood-Horse still uses this system, and one advantage is its ability to deal with foreign earnings. "The earnings of Northern Dancer's foreign runners are compared with the average per runner in that country," explained Greg Medley of the magazine's research department. "Some of our competitors use only the computerized North American data, but we manually add in the foreign earnings. The AEI works very well for handling the problem of foreign runners."

But there's still that damned Buckaroo messing up the figures. He had an out-of-sight AEI of 7.14 last year.

Richard Broadbent, the head of Lexington, Ky.-based Bloodstock Research, saw some weaknesses in the AEI and refined it with a creation he calls the Sire Production Index.

He poses this question: "You have three horses -- A, B and C -- and they all earn $100,000 in the same year. Horse A made 20 starts; B and C made 10 starts each. Horse C is a filly. Which one is the best?"

The answer is the filly, C. Obviously, a horse who makes $100,000 in 10 starts is better than one who needs 20 to get to the same figure. And because fillies earn only 39 percent of the total purse money paid in the United States, $100,000 earned by a filly is more significant than the same figure earned by a male. Broadbent's Sire Production Index takes into account these factors and measures stallions by the average earnings per start of their offspring.

Broadbent advocates using a battery of other measurements to rate a stallion, especially the percentage of his foals who win stakes and the percentage of foals who get to the races. He also looks at a stallion's record in the context of the mares to whom he has been bred -- which enables him to find underrated stallions or ones who are being "carried" by superior mares. The Blood-Horse makes a similar calculation that it calls the Comparable Index.

"When Exclusive Native had his first crop, we predicted he'd become one of the top 10 sires in North America," Broadbent said. "His dams had a Production Index of 1.41, which is what you would expect them to produce if they were bred to the average stallion. But Exclusive Native had a Sire Production Index of 3.40. Those numbers told us he had been bred to bad mares and improved them."

As useful as the SPI may be, it still contains the same flaw as every other system based on average earnings. In the list of the leading active sires, Buckaroo is No. 4 in the world -- way ahead of Northern Dancer.

Nobody frets over the imperfections in the statistical rankings of stallions more than Bill Oppenheim. His publication, Racing Update, is a mass of computer-generated breeding statistics, and Buckaroo's fluky numbers told him something he has felt for a long time: "We just can't use average earnings any more. We have got to go in another direction."

Oppenheim considered and rejected the idea of using median figures for averages; instead, he concocted a whole new measurement that he christened the Annual Progeny Earnings Index, or APEX. Its key component answers this question: How likely is a stallion to sire a foal who will earn $50,000 or more -- or its European equivalent -- in a year?

Oppenheim solves the problem presented by Europe's lower purse structure in an interesting way. Having calculated that $50,000 earners constitute 6 percent of the horses racing in North America in a year, he puts horses into this category if they are among the top 6 percent in France or Great Britain. In England, a horse qualifies if he earns 9,400 pounds, or $13,630.

If a stallion has an APEX of 1.00, he has as many $50,000 earners as the average stallion; if the index is 2.00, he has twice as many. "This is the first stallion index," Oppenheim said, "which reflects the existence of a world thoroughbred market as a unified transatlantic system. And it overcomes a bias in favor of the one-trick sire which perhaps only once in its career produces an outstanding runner." Sure enough, in the APEX ratings, Northern Dancer has a solid figure in the vicinity of 2.30, while Buckaroo is a subpar 0.75.

The weakness in the APEX is that it can give a stallion as much credit for siring a durable, high-priced New York claiming horse who wins $50,000 as it does for a winner of the Epsom Derby.

Probably, there is no way to create an absolutely perfect numerical evaluation of stallions, but with so much money at stake in the thoroughbred breeding industry, the quest will go on and on.